Epidemiology Pocket Guide

A Pocket Guide to Epidemiology

David G. Kleinbaum Kevin M. Sullivan Nancy D. Barker

A Pocket Guide to Epidemiology

David G. Kleinbaum Department of Epidemiology Rollins School of Public Health Emory University 1518 Clifton Road, NE Atlanta, GA 30322 USA [e-mail] Nancy D. Barker 2465 Traywick Chase Alpharetta, GA 30004 USA [e-mail]

Kevin M. Sullivan Department of Epidemiology Rollins School of Public Health Emory University 1518 Clifton Road, NE Atlanta, GA 30322 USA [e-mail]

Library of Congress Control Number: 2006933294 ISBN-10: 0-387-45964-2 ISBN-13: 978-0-387-45964-6 Printed on acid-free paper.
C 2007 Springer Science+Business Media, LLC All rights reserved. This work may not be translated
or copied in whole or in part without the written permission of the publisher (Springer Science+Business Media, LLC, 233 Spring Street, New York, NY 10013, USA), except for brief excerpts in connection with reviews or scholarly analysis. Use in connection with any form of information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed is forbidden. The use in this publication of trade names, trademarks, service marks, and similar terms, even if they are not identified as such, is not to be taken as an expression of opinion as to whether or not they are subject to proprietary rights.

e-ISBN-10: 0-387-45966-9 e-ISBN-13: 978-0-387-45966-0

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Preface
Four years ago (2002), I (DGK) authored a unique educational program, ActivEpi (Springer Publishers), developed in CD-ROM format to provide a multimedia interactive “electronic textbook” on basic principles and methods of epidemiology. In 2003, the ActivEpi Companion Text, authored by myself (DGK), KM Sullivan and ND Barker and also published by Springer, was developed to provide a hardcopy of the material contained in the ActivEpi CD-ROM. The CD-ROM contains 15 chapters, with each consisting of a collection of “activities” including narrated expositions, interactive study questions, quizzes, homework questions, and web links to relevant references on the Internet. In the nearly three years since the publication of the ActivEpi CD-ROM, we have received several suggestions from instructors of introductory epidemiology courses as well as health and medical professionals to produce an abbreviated version that narrows the discussion to the most “essential” principles and methods. Instructors expressed to us their concern that the material covered by the CDROM (and likewise, the Companion Text) was too comprehensive to conveniently fit the amount of time available in an introductory course. Professionals expressed their desire for a more economically time-consuming version that would conveniently fit their “after hours” availability. To address these suggestions, we have herewith produced A Pocket Guide to Epidemiology which provides a much shorter, more “essential” version of the material covered by the ActivEpi CD-ROM and Companion Text. We realize that determining what is “essential” is not a simple task, especially since, from our point of view, the original CD-ROM was already restricted to “essential” topics. Nevertheless, to produce this text, we decided to remove from the original material a great many fine points of explanation and complicated topics/issues about epidemiologic principles and methods, with our primary goal a “quicker read”. A Pocket Guide to Epidemiology contains less than half as many pages as the ActivEpi Companion Text. We have continued to include in A Pocket Guide to Epidemiology many of the study questions and quizzes that are provided in each Lesson of the CD ROM, but we have eliminated homework exercises, computer exercises, and Internet linkages from the original CD-ROM. Nevertheless, we indicate throughout A Pocket Guide to Epidemiology how and where the interested reader can turn to the ActivEpi CD ROM (or the Companion Text) to pursue more detailed information. We authors view A Pocket Guide to Epidemiology as a stand-alone introductory text on the basic principles and concepts of epidemiology. Our primary audience for this text is the public health student or professional, clinician, health journalist, and anyone else at any age or life experience that is interested in learning what epidemiology is all about in a convenient, easy to understand format with timely, real-world health examples. We believe that the reader of this text will also benefit from using the multi-media learner-interactive features of the ActivEpi CD ROM electronic textbook to further clarify and enhance what is covered in this more abbreviated (non-electronic) text. Nevertheless, we suggest that, on its own, A Pocket Guide to Epidemiology will provide the interested reader with a comfortable, time-efficient and enjoyable introduction to epidemiology.

About the Authors David G. Kleinbaum, Kevin M. Sullivan, and Nancy Barker David G. Kleinbaum is a Professor of Epidemiology at Emory University 's Rollins School of Public Health in Atlanta, GA, and an internationally recognized expert in teaching biostatistical and epidemiological concepts and methods at all levels. He is the author of several widely acclaimed textbooks including, Applied Regression Analysis and Other Multivariable Methods, Epidemiologic Research: Principles and Quantitative Methods, Logistic Regression-A Self-Learning Text, and Survival Analysis-A Self-Learning Text. Dr. Kleinbaum has more than 25 years of experience teaching over 100 short courses on statistical and epidemiologic methods to a variety of international audiences, and has published widely in both the methodological and applied public health literature. He is also an experienced and sought-after consultant, and is presently an ad-hoc consultant to all research staff at the Centers for Disease Control and Prevention. On a personal note, Dr. Kleinbaum is an accomplished jazz flutist, and plays weekly in Atlanta with his jazz combo, The Moonlighters Jazz Band. Dr. Kevin M. Sullivan is an Associate Professor of Epidemiology at Emory University’s Rollins School of Public Health. He has worked in the area of epidemiology and public health for over 30 years and has over 80 publications in peer-reviewed journals and has published chapters in several books. Dr. Sullivan has used the ActivEpi Companion Textbook and CD-ROM in a number of courses he teaches, both in traditional classroom-based courses and distance learning courses. He is one of the developers of Epi Info, a freely downloadable web-based software package for the analysis of epidemiologic data published by the Centers for Disease Control and Prevention. He is also the co-author of OpenEpi, a freely downloadable web-based calculator for epidemiologic data (www.OpenEpi.com). Ms. Nancy Barker is a statistical consultant who formerly worked at the Centers for Disease Control and Prevention. She is an Instructor in the Career MPH at the Rollins School of Public Health at Emory University where she teaches a distance learning course on Basic Epidemiology that uses ActivEpi CD and ActivEpi Companion Text as the course textbooks. She also has been coinstructor in several short courses on beginning and intermediate epidemiologic methods in the Epi in Action Program sponsored by the Rollins School of Public Health.

Contents
Preface Chapter 1 Chapter 2 Chapter 3 Chapter 4 Chapter 5 Chapter 6 Chapter 7 Chapter 8 Chapter 9 Chapter 10 . . . . . . . . . v 1 3 21 A Pocket-Size Introduction.

The Big Picture – With Examples

How to Set Things Up? Study Designs .

How Often does it Happen? Disease Frequency 43 What’s the Answer? Measures of Effect. What is the Public Health Impact? . 71 91 109 127 139

Is There Something Wrong? Validity & Bias Were Subjects Chosen Badly? Selection Bias Are the Data Correct? Information Bias . Other Factors Accounted For? Confounding Interaction . . . .

161

Chapter 11

Confounding can be Counfounding – Several Risk Factors . . . . 179

Chapter 12

Simple Analyses – 2x2 Tables are Not That Simple . . . . . . . 191 227

Chapter 13 Chapter 14

Control – What it’s All About

How to Deal with Lots of Tables - Stratified Analysis . . . . 245 257 277

Chapter 15 Index

Matching – Seems Easy, but not That Easy. . . . . . .

CHAPTER

1

A POCKET-SIZE INTRODUCTION
Epidemiology is the study of health and illness in human populations. For example, a randomized clinical trial conducted by Epidemiologists at the Harvard School of Public Health showed that taking aspirin reduces heart attack risk by 20 to 30 percent. Public health studies in the 1950 's demonstrated that smoking cigarettes causes lung cancer. Environmental epidemiologists have been evaluating the evidence that living near power lines may have a high risk for childhood leukemia. Cancer researchers wonder why older women are less likely to be screened for breast cancer than younger women. All of these are examples of epidemiologic research, because they all attempt to describe the relationship between a health outcome and one or more explanations or causes of that outcome. All of these examples share several challenges: they must choose an appropriate study design, they must be careful to avoid bias, and they must use appropriate statistical methods to analyze the data. Epidemiology deals with each of these three challenges.

CHAPTER

2

THE BIG PICTURE - WITH EXAMPLES
The field of epidemiology was initially concerned with providing a methodological basis for the study and control of population epidemics. Now, however, epidemiology has a much broader scope, including the study of both acute and chronic diseases, the quality of health care, and mental health problems. As the focus of epidemiologic inquiry has broadened, so has the methodology. In this overview chapter, we describe examples of epidemiologic research and introduce several important methodological issues typically considered in such research.

The Sydney Beach Users Study
Epidemiology is primarily concerned with identifying the important factors or variables that influence a health outcome of interest. In the Sydney Beach Users Study, the key question was “Is swimming at the beaches in Sydney associated with an increased risk of acute infectious illness?” In Sydney, Australia, throughout the 1980s, complaints were expressed in the local news media that the popular public beaches surrounding the city were becoming more and more unsafe for swimming. Much of the concern focused on the suspicion that the beaches were being increasingly polluted by waste disposal. In 1989, the New South Wales Department of Health decided to undertake a study to investigate the extent to which swimming and possible pollution at 12 popular Sydney beaches affected the public 's health, particularly during the s Summer months when the beaches were most crowded. The primary research question of interest was: are persons who swim at Sydney beaches at increased risk for developing an acute infectious illness? The study was carried out by selecting subjects on the beaches throughout the summer months of 1989-90. Those subjects eligible to participate at this initial interview were then followed-up by phone a week later to determine swimming exposure on the day of the beach interview and subsequent illness status during the week following the interview. Water quality measurements at the beaches were also taken on each day that subjects were sampled in order to match swimming exposure to pollution levels at the beaches. Analysis of the study data lead to the overall conclusion that swimming in polluted water carried a statistically significant 33% increased risk for an infectious illness when compared to swimming in non-polluted water. These

4 Chapter 2. The Big Picture – With Examples results were considered by health department officials and the public alike to confirm that swimming in Sydney beaches posed an important health problem. Consequently, the state and local health departments together with other environmental agencies in the Sydney area undertook a program to reduce sources of pollution of beach water that lead to improved water quality at the beaches during the 1990’s. Summary The Sydney Beach Users Study is an example of the application of epidemiologic principles and methods to investigate a localized public health issue. The key question in the Sydney Beach Users Study was: o Does swimming at the beaches in Sydney, Australia (in 1989-90) pose an increased health risk for acute infectious illnesses? o The conclusion was yes, a 33% increased risk.

Important Methodological Issues
We provide a general perspective of epidemiologic research by highlighting several broad issues that arise during the course of most epidemiologic investigations. There are many issues to worry about when planning an epidemiologic research study (see Box below). In this chapter we will begin to describe a list of broad methodological issues that need to be addressed. We will illustrate each issue using the previously described Sydney Beach Users Study of 1989. Issues to consider when planning an epidemiologic research study
Question Variables Design Frequency Effect Bias Analysis Define a question of interest and key variables What to measure: exposure (E), disease (D), and control (C) variables What study design and sampling frame? Measures of disease frequency Measures of effect Flaws in study design, collection, or analysis Perform appropriate analyses

The first two issues require clearly defining the study question of interest, followed by specifying the key variables to be measured. Typically, we first should ask: What is the relationship of one or more hypothesized determinants to a disease or health outcome of interest? A determinant is often called an exposure variable and is denoted by the letter E. The disease or health outcome is often denoted as D. Generally, variables other than exposure and disease that are known to predict the health outcome must be taken into account. We often call these variables control variables and denote them using the letter C. Next, we must determine how to actually measure these variables. This step requires determining the information-gathering instruments and survey questionnaires to be obtained or developed.

A Pocket Guide to Epidemiology 5 The next issue is to select an appropriate study design and devise a sampling plan for enrolling subjects into the study. The choice of study design and sampling plan depends on feasibility and cost as well as a variety of characteristics of the population being studied and the study purpose. Measures of disease frequency and effect then need to be chosen based on the study design. A measure of disease frequency provides quantitative information about how often a health outcome occurs in subgroups of interest. A measure of effect allows for a comparison among subgroups. We must also consider the potential biases of a study. Are there any flaws in the study design, the methods of data collection, or the methods of data analysis that could lead to spurious conclusions about the exposure-disease relationship? Finally, we must perform the appropriate data analysis, including stratification and mathematical modeling as appropriate. Analysis of epidemiologic data often includes taking into account other previously known risk factors for the health outcome. Failing to do this can often distort the results and lead to incorrect conclusions. Summary: Important Methodological Issues What is the study question? How should the study variables be measured? How should the study be designed? What measures of disease frequency should be used? What kinds of bias are likely? How do we analyze the study data? Data is obtained from:
Surveys Interviews Samples Laboratories

Terms to learn Study Designs:
Clinical trials Cross-sectional Case-control Cohort Measures of Disease Frequency Rate Proportion Risk Odds Prevalence Incidence Measures of Effect Risk ratio Odds ratio Rate ratio Prevalence ratio Biases Selection bias Information bias Confounding bias Data Analysis Logistic Regression Risk factors Confounding Effect modification

The Study Question
Epidemiology is primarily concerned with identifying the important factors or variables that influence a health outcome of interest. Therefore, an important first step in an epidemiologic research study is to carefully state the key study question of interest. The study question needs to be stated as clearly and as early as possible, particularly to indicate the variables to be observed or measured. A typical epidemiologic research question describes the relationship between a health outcome variable, D, and an exposure variable, E, taking into account the effects of other variables already known to predict the outcome (C, control variables).

6 Chapter 2. The Big Picture – With Examples A simple situation, which is our primary focus throughout the course, occurs when there is only one D and one E, and there are several control variables. Then, the typical research question can be expressed as shown below, where the arrow indicates that the variables E and the controls (Cs) on the left are the variables to be evaluated as predictors of the outcome D, shown on the right. In the Sydney Beach Users Study, the health outcome variable, D, of interest is whether or not a person swimming at a beach in Sydney develops an acute infectious illness such as a cough, cold, flu, ear infection, or eye infection, within one week of swimming at the beach. The study subjects could be classified as either: D=0 for those did not get ill, or D=l for those became ill. A logical choice for the exposure variable is the exposure variable swimming status, which is set to: E=0 for non-swimmers and E=1 for swimmers during the time period of the study. (Note that other coding schemes could be used other than 0/1, such as 1/2, Y/N, or +/-, but we will use 0/1). Control variables might include pollution level at the beach, age of the subject, and duration of swimming. Generally speaking, a study will not be very useful unless a question or hypothesis of some kind can be formulated to justify the time and expense needed to carry out the study. Thus, the research question of this study example is to describe the relationship of swimming to the development of an infectious illness, while taking into account the effects of relevant control variables such as pollution level, age of subject and duration of swimming. Because several variables are involved, we can expect that a complicated set of analyses will be required to deal with all the possible relationships among the variables involved. Summary: The Study Question An important first step in an epidemiologic research study is to carefully state the key study question of interest. The general question: To what extent is there an association between one or more exposure variables (Es) and a health outcome (D), taking into account (i.e., controlling for) the possible influence of other important covariates (Cs)? We can expect a complicated set of analyses to be required to deal with all possible relationships among the variables involved. Quiz (Q2.1) In the Sydney Beach Users study, exposure was alternatively defined by distinguishing those who swam in polluted water from those who swam in nonpolluted water and from those who did not swim at all. Based on this scenario, fill

A Pocket Guide to Epidemiology 7 in the missing information in the following statement: 1. The exposure variable has ??? categories, one of which is ??? Choices: 2 3 4 5 did not swim polluted water swam water not polluted 2. When considering both swimming and pollution together, which of the following choices is appropriate for defining the exposure variable in the Sydney Beach Users study: ??? Choices: a) E=O if did not swim, E=1 if swam in polluted water b) E=O if did not swim, E=1 if swam in non-polluted water c) E=O if did not swim, E=1 if swam in polluted water, E=2 if swam in nonpolluted water d) E=O if did not swim, E=1 if swam In the Sydney Beach Users study, the illness outcome was whether or not an acute infectious illness developed 1 week after swimming at the beach. Also, in addition to age, another control variable was whether or not a study subject swam on days other than the day he or she was interviewed. Fill in the missing information: 3. The health outcome has ??? categories. 4. There are at least ??? control variables. 5. Which of the following choices is not a control variable: ??? a) age b) swimming status on other days c) swimming status on day of interview Choices: 2 3 4 5 a b c

Measuring the Variables
Another important issue is: How do we measure the variables to be studied? Several measurement issues are now introduced. Once the study question is determined, the investigators must determine how to measure the variables identified for the study and any other information that is needed. For example, how will the exposure variable be measured? If a subject went into the water but never put his head under the water, does that count as swimming? How much time is required to spend in the water to be counted as swimming? Is it feasible to observe each subject 's swimming status on the day of initial interview, and if not, how should swimming status be determined? After considering these Measuring Exposure Variables questions, the study team defined Definition of Swimming swimming as any immersion of the Any immersion of face & head in water face and head in the water. It was Measuring Swimming Status decided that subject self-reporting Subject self-reporting of swimming was the only feasible way to obtain swimming information. How will the health outcome be measured? Should illness be determined by a subject 's self-report, which might be inaccurate, or by a physician 's confirmation, which might not be available? The study team decided to use self-reported

8 Chapter 2. The Big Picture – With Examples symptoms of illness obtained by telephone interview of study subjects 7 to 10 days after the initial interview. Another measurement issue concerned how to determine water quality at the beach. Do water samples need to be collected? What time of day should they be collected? How will such information be linked to study subjects? The study team decided that health department surveyors would collect morning and evening samples at the midpoint of each of three sectors of the beach. As nearly as could practicably be achieved, study subjects were to be interviewed during the period in which water samples were taken. A standard protocol was determined for how much water was to be sampled and how samples were to be assessed for water quality. A final measurement issue concerned what Interview Variables information should be obtained from persons Age interviewed at the beach for possible inclusion into the Sex study? The study team decided to collect basic Postcode Swimming history demographic data including age, sex, and postcode, to Health status ask whether or not each respondent had been swimming anywhere in the previous 5 days, and had any condition that precluded swimming on the day of the interview. Subjects were excluded from the study if they reported swimming in the previous 5 days or having an illness that prevented them from swimming. Subjects were included if they were at least 15 years old and agreed to both an initial beach interview and a follow-up telephone interview. All the measurement issues described above must be addressed prior to data collection to ensure standardized information is collected and to provide a study that is both cost and time efficient. Study Questions (Q2.2) 1. What other variables might you also consider as control variables in the Beach Users Study? 2. How do we decide which variables to measure as control variables? 3. Why should age be considered? 4. How would you deal with subjects who went to the beach on more than one day? Summary: Measuring the Variables General measurement issues: How to operationalize the way a measurement is carried out? Should self-reporting of exposure and/or health outcome be used? When should measurements be taken? How many measurements should be taken on each variable and how should several measurements be combined? How to link environmental measures with individual subjects?

A Pocket Guide to Epidemiology 9

The Study Design, including the Sampling Plan
Another important issue is: What study design should be used and how should we select study subjects? Several study design issues are now introduced. There are a variety of study designs used in epidemiology. The Sydney Beach Users study employed a cohort design. A key feature of such a design is that subjects without the health outcome are followed-up over time to determine if they develop the outcome. Subjects were selected from 12 popular Sydney beaches over 41 sampling days. An initial interview with the study subjects took place on the beach to obtain consent to participate in the study and to obtain demographic information. Persons were excluded from the study if they had an illness that prevented them from swimming on that day or if they had been swimming within the previous 5 days. It was not considered feasible to determine swimming exposure status of each subject on the day of initial interview. Consequently, a follow-up telephone interview was conducted 7 to 10 days later to obtain self-reported swimming exposure as well as illness status of each subject. Study Questions (Q2.3) 1. How might you criticize the choice of using self-reported exposure and illnesses? 2. How might you criticize the decision to determine swimming status from a telephone interview conducted 7 to 10 days after being interviewed on the beach? A complex sample survey design was used to obtain the nearly 3000 study participants. Six beaches were selected on any given day and included 2 each from the northern, eastern and southern areas of Sydney. Each beach was divided into three sectors, defined by the position of the swimming area flags erected by the lifeguards. Trained interviewers recruited subjects, starting at the center of each sector and moving in a clockwise fashion until a quota for that sector had been reached. Potential subjects had to be at least 3 meters apart. Study Questions (Q2.4) 1. Why do you think potential subjects in a given sector of the beach were specified to be at least 3 meters apart? 2. Why is the Sydney Beach Users Study a cohort study? 3. A fixed cohort is a group of people identified at the onset of a study and then followed over time to determine if they developed the outcome. Was a fixed cohort used in the Sydney Beach Users Study? Explain. 4. A case-control design starts with subjects with and without an illness and looks back in time to determine prior exposure history for both groups. Why is the Sydney Beach Users study not a case-control study? 5. In a cross-sectional study, both exposure and disease status are observed at the same time that subjects are selected into the study. Why is the Sydney Beach Users study not a cross-sectional study?

10 Chapter 2. The Big Picture – With Examples Summary: Study Design Two general design issues: o Which of several alternative forms of epidemiologic study designs should be used (e.g., cohort, case-control, cross-sectional)? o What is the sampling plan for selecting subjects?

Measures of Disease Frequency and Effect
Another important issue is: What measure of disease frequency and measure of effect should be used? These terms are now briefly introduced. Once the study design has been determined, appropriate measures of disease frequency and effect can be specified. A measure of disease frequency provides quantitative information about how often the health outcome has occurred in a subgroup of interest. For example, in the Sydney Beach Users Study, if we want to measure the frequency with which those who swam developed the illness of interest, we could determine the number of subjects who got ill and swam and divide by the total number who swam. The denominator represents the total number of study subjects among swimmers that had the opportunity to become ill. The numerator gives the number of study subjects among swimmers who actually became ill. Similarly, if we want to measure the frequency of illness among those who did not swim, we could divide the number of subjects who got ill and did not swim by the total number of non-swimming subjects. The information required to carry out the above calculations can be described in the form of a two-way table shown below. This table shows the number who became ill among swimmers and nonswimmers. We can calculate the proportion ill among the swimmers to be 0.277 or 27.7 percent. We can also calculate the proportion ill among the non-swimmers as 0.165 or 16.5 percent. Each proportion is a measure of disease frequency called a risk. R(E) denotes the risk among the exposed for developing the health outcome. R(not E) [or R( E )] denotes the risk among the unexposed. There are measures of disease frequency other than risk that will be described in this course. The choice of measure (e.g., risk, odds, prevalence, or rate) primarily depends on the type of study design being used and the goal of the research study.

A Pocket Guide to Epidemiology 11 If we want to compare two measures of disease frequency, such as two risks, we can divide one risk by the other, say, the risk for swimmers divided by the risk for non-swimmers. We find that the ratio of these risks in our study is 1.68; this means that swimmers have a risk for the illness that is 1.68 times the risk for nonswimmers. Such a measure is called a measure of effect. In this example, the effect of interest refers to the effect of one 's swimming status on becoming or not becoming ill. If we divide one risk by the other, the measure of effect or association is called a risk ratio. There are other measures of effect that will be described in this course (e.g., such as the risk ratio, odds ratio, prevalence ratio, rate ratio, risk difference, and rate difference). As with measures of disease frequency, the choice of effect measure depends on the type of study design and the goal of the research study. Summary: Measures of Disease Frequency and Effect A measure of disease frequency quantifies how often the health outcome has occurred in a subgroup of interest. A measure of effect quantifies a comparison of measures of disease frequency for two or more subgroups. The choice of measure of disease frequency and measure of effect depends on the type of study design used and the goal of the research study.

Bias
Another important issue is: What are the potential biases of the study? The concept of bias is now briefly introduced. The next methodologic issue concerns Bias: A flaw in the the potential biases of a study. Bias is a o study design flaw in the study design, the methods of o methods of data collection data collection, or the methods of data o methods of data analysis analysis that may lead to spurious … which leads to spurious conclusions. conclusions about the exposure-disease Sources of bias: relationship. Bias may occur because of: o Selection o Information the selection of study subjects; incorrect o Confounding information gathered on study subjects; or failure to adjust for variables other than the exposure variable, commonly called confounding. In the Sydney Beach Users Study, all 3 sources of bias were considered. For example, to avoid selection bias, subjects were excluded from the analysis if they were already ill on the day of the interview. This ensured that the sample represented only those healthy enough to go swimming on the day of interview. Sometimes selection bias cannot be avoided. For example, subjects had to be excluded from the study if they did not complete the follow-up interview. This

12 Chapter 2. The Big Picture – With Examples non-response bias may affect how representative the sample is. There was also potential for information bias since both swimming status and illness status were based on self-reporting by study subjects. Swimming status was determined by self-report at least seven days after the swimming occurred. Also, the report of illness outcome did not involve any clinical confirmation of reported symptoms. Confounding in the Beach Users Study concerned whether all relevant variables other than swimming status and pollution level exposures were taken into account. Included among such variables were age, sex, duration of swimming for those who swam, and whether or not a person swam on additional days after being interviewed at the beach. The primary reason for taking into account such variables was to ensure that any observed effect of swimming on illness outcome could not be explained away by these other variables. Summary Bias is a flaw in the study design, the methods of data collection, or the methods of data analysis that may lead to spurious conclusions about the exposure-disease relationship. Three general sources of bias occur in: o Selection of study subjects o Incorrect information gathered on study subjects o Failure to adjust for variables other than the exposure variable (confounding)

Analyzing the data
Another important issue is: How do we carry out the data analysis? We now briefly introduce some basic ideas about data analysis. The final methodologic issue concerns the Statistics data analysis. We must carry out an Frequency Effect Risk risk ratio appropriate analysis once collection and Prevalence prevalence ratio processing of the study data are complete. Odds odds ratio Since the data usually come from a sample of Rate rate ratio subjects, the data analysis typically requires Stratification the use of statistical procedures to account for Mathematical modeling the inherent variability in the data. In epidemiology, data analysis typically begins with the calculation and statistical assessment of simple measures of disease frequency and effect. The analysis often progresses to more advanced techniques such as stratification and mathematical modeling. These latter methods are typically used to control for one or more potential confounders. Let 's consider the data analysis in the Sydney Beach Users Study. We had previously compared swimmers with non-swimmers. Now, we may wish to address the more specific question of whether those who swam in polluted water had a higher risk for illness than those who swam in non-polluted water. We can do this by separating the swimmers into two groups. The non-swimmers represent a baseline comparison group with which the two groups of swimmers can be compared.

A Pocket Guide to Epidemiology 13 Based on the two-way table, we can estimate the risk for illness for each of the three groups by computing the proportion that got ill out of the total for each group. The three risk estimates are 0.357, 0.269 and 0.165, which translates to 35.7 percent, 26.9 percent and 16.5 percent, respectively. The risk ratio that compares the Swam-Polluted (Yes-P) group with the Swam-Nonpolluted (Yes-NP) group is 1.33 indicating that persons who swam in polluted water had a 33 percent increased risk than persons who swam in nonpolluted water. Also, the risk ratio estimates obtained by dividing the risks for each group by risk for non-swimmers are 2.16, 1.63, and 1. This suggests what we call a dose-response effect, which means that as exposure increases, the risk increases. The analysis just described is called a “crude” analysis because it does not take into account the effects of other known factors that may also affect the health outcome being studied. A list of such variables might include age, swimming duration, and whether or not a person swam on additional days. The conclusions found from a crude analysis might be altered drastically after adjusting for these potentially confounding variables. Several questions arise when considering the control of many variables: Which of the variables being considered should actually be controlled? What is gained or lost by controlling for too many or too few variables? What should we do if we have so many variables to control that we run out of numbers? What actually is involved in carrying out a stratified analysis or mathematical modeling to control for several variables? How do the different methods for control, such as stratification and mathematical modeling, compare to one another? These questions will be addressed in later activities. Study Questions (Q2.5) 1. How do you interpret the risk ratio estimate of 1.33? 2. Does the estimated risk ratio of 1.33 indicate that swimming in polluted water poses a health risk? 3. Given the relatively small number of 154 persons who swam in polluted water, what statistical question would you need to answer about the importance of the estimated risk ratio of 1.33? Summary: Analyzing the Data The data analysis typically requires the use of statistical procedures to account for the inherent variability in the data.

14 Chapter 2. The Big Picture – With Examples In epidemiology, data analysis often begins with assessment and comparison of simple measures of disease frequency and effect. The analysis often progresses to more advanced techniques such as stratification and mathematical modeling.

Example: Alcohol Consumption and Breast Cancer
The Harvard School of Public Health followed a cohort of about 100,000 nurses from all over the US throughout the 1980s and into the 1990s. The investigators in this Nurses Health Study, were interested in assessing the possible relationship between diet and cancer. One particular question concerned the extent to which alcohol consumption was associated with the development of breast cancer. Nurses identified as being 'disease free ' at enrollment into the study were asked about the amount of alcohol they currently drank. Other relevant factors, such as age and smoking history, were also determined. Subjects were followed for four years, at which time it was determined who developed breast cancer and who did not. A report of these findings was published in the New England Journal of Medicine in 1987. Recall that the first methodologic issue is to define the study question. Which of the study questions stated here best addresses the question of interest in this study? A. Is there a relationship between drinking alcohol and developing breast cancer? B. Are alcohol consumption, age, and smoking associated with developing breast cancer? C. Are age and smoking associated with developing breast cancer, after controlling for alcohol consumption? D. Is alcohol consumption associated with developing breast cancer, after accounting for other variables related to the development of breast cancer? The best answer is "D,” although “A” is also correct. In stating the study question of interest, we must identify the primary variables to be measured. Study Questions (Q2.6) Determine whether each of the following is a: Health outcome variable (D), Exposure variable (E), or Control variable (C) 1. 2. 3. 4. Smoking history Whether or not a subject develops breast cancer during follow-up Some measure of alcohol consumption Age

Once we have specified the appropriate variables for the study, we must determine how to measure them. The health outcome variable, D, in this example is simply yes or no depending on whether or not a person was clinically diagnosed with breast cancer. The investigators at Harvard interviewed study subjects about their drinking habits, E, and came up with a quantitative

A Pocket Guide to Epidemiology 15 measurement of the amount of alcohol in units of grams per day that were consumed in an average week around the time of enrollment into the study. How to treat this variable for purposes of the analysis of the study data was an important question considered. One approach was to categorize the alcohol measurement into 'high ' versus 'low '. Another approach was to categorize alcohol into 4 groups: non-drinkers; less than 5 grams per day; between 5 and 15 grams per day; and 15 or more grams per day. Age, denoted C1, is inherently a quantitative variable, although many of the analyses treated age as a categorical variable in three age groups, shown here: 34 to 44 years, 45 to 54 years, 55 to 59 years Smoking history, C2, was categorized in several ways; one was never smoked versus ever smoked. The research question in the nurse 's health study can thus be described as determining if there is a relationship between alcohol consumption, E, and breast cancer, D, controlling for the effects of age, Cl, and smoking history, C2, and possibly other variables (C3, C4, etc.). Although a detailed analysis is not described here, the data did provide evidence of a significant association between alcohol use and development of breast cancer. For heavy drinkers, when compared to non-drinkers, there was about an 80% increase in the risk of developing breast cancer. Moderate drinkers were found to have about a 50% increase in risk, and light drinkers had an increased risk of about 20%. Note: The Nurses Health Study provides an example in which the exposure variable, alcohol consumption, has several categories rather than simply binary. Also, the control variables age and smoking history can be a mixture of different types of variables. In the Nurses Health Study, age is treated in three categories, and smoking history is treated as a binary variable.

Example: The Bogalusa Outbreak
On October 31, 1989, the Louisiana State Health Department was notified by two physicians in Bogalusa, Louisiana, that over 50 cases of acute pneumonia had occurred within a three-week interval in mid to late October, and that six persons had died. Information that the physicians had obtained from several patients suggested that the illness might have been Legionnaires Disease. In 1989, Bogalusa was a town of about 16,000 persons. The largest employer was a paper mill located in the center of town adjacent to the main street. The

16 Chapter 2.
The Big Picture – With Examples paper mill included five prominent cooling towers. The mill also had three paper machines that emitted large volumes of aerosol along the main street of town. Many people suspected that the cooling towers and or the paper mill were the cause of the outbreak, since they were prominent sources of outdoor aerosols where the legionnaire 's bacteria could have been located. Recall that the first methodologic issue is to define the study question of interest. Which of the study questions stated here best addresses the question of interest in this study? A. Was the paper mill the source of the Legionnaires Disease outbreak in Bogalusa? B. What was the source of the outbreak of Legionnaires Disease in Bogalusa? C. Why did the paper mill cause the outbreak of Legionnaires Disease in Bogalusa? D. Was there an outbreak of Legionnaires Disease in Bogalusa? The most appropriate study question is "B." Even though the paper mill was the suspected source, the study was not limited to that variable only, otherwise, it might have failed to collect information on the true source of the outbreak. Study Questions (Q2.7) In stating the study question, we identify the primary variables to be considered in the study. Determine whether each of these variables is the health outcome variable, D, an exposure variable, E, or a control variable, C: 1. Exposure to the cooling towers of the paper mill? 2. Exposure to emissions of the paper machines? 3. Age of subject? 4. Visited grocery store A? 5. Visited grocery store B? 6. Diagnosed with Legionnaires Disease? 7. Visited drug store A? 8. Visited drug store B? 9. Ate at restaurant A? The health outcome variable, D, indicates whether or not a study subject was clinically diagnosed with Legionnaires Disease during the three week period from mid to late October. The exposure variable is conceptually whatever variable indicates the main source of the outbreak. Since this variable is essentially unknown
at the start of the study, there is a large collection of exposure variables, all of which need to be identified as part of the study design and investigated as candidates for being the primary source of the outbreak. We denote these exposure variables of interest E1 through E7. One potential control variable of interest was age, which we denoted as C1. The general research question of interest in the Bogalusa outbreak was to evaluate the relationship of one or more of the exposure variables to whether or not a study subject developed Legionnaires Disease, controlling for age. A case-control study, was carried out in which 28 cases diagnosed with confirmed Legionnaires Disease were compared with 56 non-cases or controls. This investigation led to the hypothesis that a misting machine for vegetables in a grocery store was the source of the outbreak. This misting machine was removed

A Pocket Guide to Epidemiology 17 from the grocery store and sent to CDC where laboratory staff was able to isolate Legionella organisms from aerosols produced by the machine. This source was a previously unrecognized vehicle for the transmission of Legionella bacteria. Note: The Bogalusa study provides an example in which there are several exposure variables that are candidates as the primary source of the health outcome being studied. Hopefully, the investigators will be able to identify at least one exposure variable as being implicated in the occurrence of the outbreak. It is even possible that more than one candidate exposure variable may be identified as a possible source. The case-control study of this and many other outbreaks can often be viewed as hypothesis generating. Further study, often using laboratory methods, clinical diagnosis, and environmental survey techniques, must often be carried out in order to confirm a suspected exposure as the primary source of the outbreak. The Centers for Disease Control and Prevention has a variety of scientists to provide the different expertise and teamwork that is required, as carried out in the Bogalusa study.

Example: The Rotterdam Study
The Rotterdam study has been investigating the determinants of chronic disabling diseases, including Alzheimer’s disease, during the 1990s and beyond. In the early 1990s, the Department of Epidemiology of the Erasmus University in Rotterdam, the Netherlands, initiated the Rotterdam Study. A cohort of nearly 8000 elderly people was selected. They continue to be followed to this day. The goal of the study is to investigate determinants of chronic disabling diseases, such as Alzheimer 's and cardiovascular disease. One particular study question of interest was whether smoking increases the risk of Alzheimer 's disease. Subjects who were free of dementia at a first examination were included in the study. This excluded anyone diagnosed at this exam with Alzheimer 's or any other form of dementia due to organic or psychological factors. Approximately two years later, the participants were asked to take a brief cognition test. If they scored positive, they were further examined by a neurologist. The investigators could then determine whether or not a participant had developed Alzheimer 's disease, the health outcome variable D of interest, since the start of follow-up. The primary exposure variable, E, was smoking history. Three categories of

18 Chapter 2. The Big Picture – With Examples smoking were considered: current smokers at the time of the interview; previous but not current smokers; and, never smokers. Control variables considered in this study included age, gender, education, and alcohol consumption. We define the study question of interest as: Is there a relationship between smoking history and Alzheimer 's disease, controlling for the effects of age, gender, education and alcohol consumption? Recall that one important methodologic issue is to determine the study design. How would you define the design of this study? 1. Cohort design 2. Case-control design 3. Cross-sectional design 4. Clinical trial This is a cohort design because participants without the health outcome of interest, in this case Alzheimer 's disease, are followed up over time to determine if they develop the outcome later in life. Which of the following is influenced by the design of the study? A. The assessment of confounding B. The choice of the measures of disease frequency and effect C. A decision regarding the use of stratified analysis D. The analysis is not influenced in any way by the study design used The answer is B. We determine the appropriate measures of disease frequency and effect based on the study design characteristics. Choices A and C are incorrect because they are typically considered regardless of the study design used. The investigators found that 105 subjects developed Alzheimer’s disease. After taking the control variables into account, the risk of Alzheimer 's disease for current smokers was 2.3 times the risk for subjects who had never smoked. For subjects who had smoked in the past but who had given up smoking before the study started, the risk of Alzheimer 's disease was 1.3 times the risk for subjects who had never smoked. Study Questions (Q2.8) Based on the above results: 1. What is the percent increase in the risk for current smokers when compared to the risk for never smokers? 2. What is the percent increase in the risk for previous smokers when compared to the risk for never smokers? Because these results were statistically significant and controlled for previously established predictors of Alzheimer 's, the study gave support to the hypothesis that smoking history was a significant risk factor in the development of Alzheimer 's disease.

A Pocket Guide to Epidemiology 19

Nomenclature
C D E R(E) R(not E) or R( E ) RR Control variable or covariate Disease or outcome variable Exposure variable Risk among the exposed for developing the health outcome Risk among nonexposed for developing the health outcome Risk ratio

References
For the Sydney Beach Users Study: Corbett SJ, Rubin GL, Curry GK, Kleinbaum DG. The health effects of swimming at Sydney Beaches. The Sydney Beach Users Study Advisory Group. Am J Public Health. 1993;83(12): 1701-6. For the Nurses Health Study: Willett WC, Stampfer MJ, Colditz GA, Rosner BA, Hennekens CH, Speizer FE. Moderate alcohol consumption and the risk of breast cancer. N Engl J Med. 1987;316(19):1174-80. For the Bogalusa Outbreak: Mahoney FJ, Hoge CW, Farley TA, Barbaree JM, Breiman RF, Benson RF, McFarland LM. Communitywide outbreak of Legionnaires ' disease associated with a grocery store mist machine. J Infect Dis. 1992;165(4):736-9. For The Rotterdam Study: Hofman A, Grobbee DE, de Jong PT, van den Ouweland FA. Determinants of disease and disability in the elderly: the Rotterdam Elderly Study. Eur J Epidemiol. 1991;7(4); 403-22. Ott A, Slooter AJ, Hofman A, van Harksamp F, Witteman JC, Van Broeckhoven C, van Duijin CM, Breteler MM. Smoking and risk of dementia and Alzheimer 's disease in a population-based cohort study: the Rotterdam Study. Lancet. 1998;351(9119):1840-3. A CDC Website. The Centers for Disease control has a website called EXCITE, which stands for Excellence in Curriculum Integration through Teaching Epidemiology. The website address is http://www.cdc.gov/excite/ We suggest that you open up this website on your computer and look over the various features and purposes of the website described on the first page you see. Then click on the item (on menu on left of page) Disease Detectives at Work and read the first two articles entitled Public Health on Front Burner After Sept 11 and USA’s ‘Disease Detectives’ Track Epidemics Worldwide. Then click on the item (on menu on left of page) Classroom Exercises and go through the exercise on Legionnaires Disease in Bogalusa, Louisiana. The specific website address for this exercise is: http://www.cdc.gov/excite/legionnaires.htm

Answers to Study Questions and Quizzes
Q2.1 1. 3, did not swim 2. C 3. 2 4. 2 5. C Q2.2 1. General health status, smoking status, diet, including what a subject might have eaten at the beach. 2. Choose variables that are already known determinants of the health outcome. This will be discussed later under the topic of confounding. 3. Younger subjects might be less likely to get ill than older subjects. 4. In the actual study, the investigators chose to exclude subjects from the analysis if they visited the beach on days other than the day they were interviewed on the beach. Q2.3 1. Self-reported information may be inaccurate and can therefore lead to spurious study results.

20 Chapter 2. The Big Picture – With Examples
2. As with the previous question, the information obtained about exposure much later than when the actual exposure occurred may be inaccurate and can lead to spurious study results. Q2.4 1. To minimize the inclusion in the study of a family or social groups. 2. Subjects without the health outcome, that is, healthy subjects selected at the beach, were followed-up over time to determine if they developed the outcome. 3. No, the Sydney Beach User’s Study did not use a fixed cohort. Study subjects were added over the summer of 198990 to form the cohort. 4. Because the study started with exposed and unexposed subjects, rather than ill and not-ill subjects, and went forward rather than backwards in time to determine disease status. 5. Exposure and disease status were observed at different times for different subjects. Also, each subject was selected one week earlier than the time his or her exposure and disease status were determined. Q2.5 1. The risk of illness for persons who swam in polluted water is estimated to be 1.33 times the risk of illness for persons who swam in non-polluted water. 2. Not necessarily. The importance of any risk ratio estimate depends on the clinical judgment of the investigators and the size of similar risk ratio estimates that have been found in previous studies. 3. Is the risk ratio of 1.33 significantly different from a risk ratio of 1? That is, could the risk ratio estimate of 1.33 have occurred by chance? Q2.6 1. C 2. D 3. E 4. C Q2.7 1. E 2. E 3. E 4. E 5. D 6. E 7. E 8. E Q2.8 1. The increased risk of 2.3 translates to a 130% increase in the risk of current smokers compared to never smokers. 2. The increased risk of 1.3 translates to a 30% increase in the risk for previous smokers compared to never smokers.

CHAPTER

3

HOW TO SET THINGS UP? STUDY DESIGNS
A key stage of epidemiologic research is the study design. This is defined to be the process of planning an empirical investigation to assess a conceptual hypothesis about the relationship between one or more exposures and a health outcome. The purpose of the study design is to transform the conceptual hypothesis into an operational hypothesis that can be empirically tested. Since all study designs are potentially flawed, it is therefore important to understand the specific strengths and limitations of each design. Most serious problems or mistakes at this stage cannot be rectified in subsequent stages of the study.

Types of Epidemiologic Research
There are two broad types of epidemiologic studies, experimental and observational. An experimental study uses randomization to allocate subjects to different categories of the exposure. An observational study does not use randomization. (For additional information on randomization, please refer to the end of this chapter.) In experimental studies, the investigator, through randomization, determines the exposure status for each subject, then follows them and documents subsequent disease outcome. In an observational study, the subjects themselves, or perhaps their genetics, determine their exposure, for example, whether to smoke or not. The investigator is relegated to the role of simply observing exposure status and subsequent disease outcome. Experimental studies in epidemiology usually take the form of clinical trials and community intervention trials. The objective of most clinical trials is to test the possible effect, that is, the efficacy, of a therapeutic or preventive treatment such as a new drug, physical therapy or dietary regimen for either treating or preventing the occurrence of a disease. The objective of most community intervention trials is to assess the effectiveness of a prevention program. For example, one might study the effectiveness of fluoridation, of sex education, or of needle exchange. Most epidemiologic studies are observational. Observational studies are broadly identified as two types: descriptive and analytic. Descriptive studies are performed to describe the natural history of a disease, to determine the allocation of health care resources, and to suggest hypotheses about disease causation. Analytic studies are performed to test hypotheses about the determinants of a disease or other health condition, with the ideal goal of assessing causation. (See the end of this chapter for additional information on disease causation.) Summary There are two broad types of epidemiologic studies: experimental and observational Experimental studies use randomization of exposures Observational studies do not use randomization of exposures

22 Chapter 3. How to Set Things Up? Study Designs In experimental studies, the investigator pro-actively determines the exposure status for each subject. In observational studies, the subject determines his/her exposure status. Experimental studies are usually clinical trials or community intervention trials. Observational studies are either descriptive or analytic.
Randomization Randomization is an allocation procedure that assigns subjects into (one of the) the exposure groups being compared so that each subject has the same probability of being in one group as in any other. Randomization tends to make demographic, behavioral, genetic, and other characteristics of the comparison groups similar except for their exposure status. As a result, if the study finds any difference in health outcome between the comparison groups, that difference can only be attributable to their difference in exposure status. For example, if subjects are randomly allocated to either a new drug or a standard drug for the treatment of hypertension, then it is hoped that other factors such as age and sex might have approximately the same distribution for subjects receiving the new drug as for subjects receiving the standard drug. Actually, there is no guarantee even with randomization that the distribution of, for example age, will be the same for the two treatment groups. The investigator can always check the data to see what has happened regarding any such characteristic, providing the characteristic is measured or observed in the study. If the age distribution is found to be different between the two treatment groups, the investigator can take this into account in the analysis, for example, by stratifying on age. The advantage of randomization is what it offers with regard to those characteristics not measured in one 's study. Variables that are not measured obviously cannot be taken into account in the analysis. Randomization offers insurance, though no guarantee, that such unmeasured variables are evenly distributed among the different exposure groups. In observational studies, on the other hand, the investigator can account for only those variables that are measured, allowing more possibility for spurious conclusions because of unknown effects of important unmeasured variables. Causation In any research field involving the conduct of scientific investigations and the analysis of data derived from such investigations to test etiologic hypotheses, the assessment of causality is a complicated issue. In particular, the ability to make causal inferences in the health sciences typically depends on synthesizing results from several studies, both epidemiologic and non-epidemiologic (e.g., laboratory or clinical findings). Instigated by a governmental sponsored effort in the United States to assess the health consequences of smoking, health scientists in the late 1950 's and 1960 's began to consider defining objective criteria for evaluating causality. The particular focus of this effort was how to address causality based on the results of studies that consider exposures that cannot be randomly assigned, i.e., observational studies. In 1964, a report was published by the US Department of Health, Education and Welfare that reviewed the research findings dealing with the health effects of smoking, with the objective of assessing whether or not smoking could be identified as a "cause" of lung cancer and perhaps other diseases. The type of synthesis carried out in this report has been referred to in the 1990 's as a meta analysis, so that this report was in essence, one of the earliest examples of a meta analysis conducted in the health sciences. The 1964 document based much of its conclusions about smoking causation on a list of general criteria that was formalized by Bradford Hill and later incorporated into a famous 1971 textbook by Hill. The criteria are listed as follows: 1. Strength of the Association: The stronger the observed association, the less likely the association is due to bias; weaker associations do not provide much support to a causal

A Pocket Guide to Epidemiology 23 interpretation. 2. Dose-response Effect: If the disease frequency increases with the dose or level of exposure, this supports a causal interpretation. (Note, however, that the absence of a dose-response effect may not rule out causation from alternative explanations, such as a threshold effect.) 3. Lack of Temporal Ambiguity: The hypothesized cause must precede the occurrence of the disease. 4. Consistency of Findings: If all studies dealing with a given relationship produce similar results, a causal interpretation is advanced. (Note: Inconsistencies may be due to different study design features, so that perhaps some kind of weighting needs to be given to each study.) 5. Biological Plausibility of the Hypothesis: If the hypothesized effect makes sense in the context of current biological knowledge, this supports a causal interpretation. (Note, however, the current state of biological knowledge may be inadequate to determine biological plausibility.) 6. Coherence of the Evidence: If the findings do not seriously conflict with our understanding of the natural history of the disease or other accepted facts about disease occurrence, this supports a causal interpretation. 7. Specificity of the Association: If the study factor is found to be associated with only one disease, or if the disease is found to be associated with only one factor, a causal interpretation is supported. (However, this criterion cannot rule out a causal hypothesis, since many factors have multiple effects and most diseases have multiple causes.) Examples include vinyl chloride and angiosarcoma of the liver; DES by women and vaginal cancer in offspring. 8. Experimentation: use of experimental evidence, such as clinical trials in humans, animal models, and in vitro laboratory experiments. May support causal theories when available, but its absent does not preclude causality. 9. Analogy: when similar relationships have been shown with other exposure-disease relationships. For example, the offspring of women given DES during pregnancy were more likely to develop vaginal cancer. By analogy, it would seem possible that other drugs given to pregnant women could cause cancer in their offspring.

Quiz (Q3.1) Fill in the blanks with Experimental or Observational 1. A strength of the ??? study is the investigator 's control in the assignment of individuals to treatment groups. 2. A potential advantage of an ??? study is that they are often carried out in more natural settings, so that the study population is more representative of the target population. 3. The major limitation of ??? studies is that they afford the investigator the least control over the study situation; therefore, results are generally more susceptible to distorting influences. 4. A weakness of an ??? study is that randomization to treatment groups may not be ethical if an arbitrary group of subjects must be denied a treatment that is regarded as beneficial. 5. One community in a state was selected by injury epidemiologists for a media campaign and bicycle helmet discount with any bicycle purchase. A similar community about 50 miles away was identified as a comparison community. The epidemiologists compared the incidence of bicycle-related injuries through emergency room surveillance and telephone survey. This is an example of an ??? study.

24 Chapter 3. How to Set Things Up? Study Designs 6. Researchers administered a questionnaire to all new students at a large state university. The questionnaire included questions about behaviors such as seat belt use, exercise, smoking, and alcohol consumption. The researchers plan to distribute follow-up questionnaires at graduation and every five years thereafter, asking about health events and conditions such as diabetes and heart disease. This is an example of an ??? study.

Directionality
The directionality of a study refers to when the exposure variable is observed relative in time to when the health outcome is observed. In a study with forward directionality, the investigator starts by determining the exposure status for subjects selected from some population of interest and then follows these subjects over time to determine whether or not they develop the health outcome. Cohort studies and clinical trials always have forward directionality. In a backwards design, the investigator selects subjects on the basis of whether or not they have the health outcome of interest, and then obtains information about their previous exposures. Case-control studies always have backwards directionality. In a non-directional design, the investigator observes both the study factor and the health outcome simultaneously, so that neither variable may be uniquely identified as occurring first. A cross-sectional study is always non-directional. The directionality of a study affects the researcher 's ability to distinguish antecedent from consequent. This is important for evaluating causality. Also, the directionality chosen affects the way subjects can be selected into the study. Designs that are backwards or non-directional have more potential for selection bias than forward designs. Selection bias will be addressed in more detail in a later chapter. Summary Directionality answers the question: when did you observe the exposure variable relative in time to when you observed health outcome? Directionality can be forward, backward, or non-directional. Directionality affects the researcher’s ability to distinguish antecedent from consequent. Directionality also affects whether or not a study will have selection bias.

A Pocket Guide to Epidemiology 25

Timing
Timing concerns the question of whether the health outcome of interest has already occurred before the study actually began. If the health outcome has occurred before the study is initiated, the timing is retrospective. For example, let’s say a case-control study is initiated to investigate cases of a disease that occurred in the previous year; this would be an example of a retrospective case control study. If, on the other hand, the health outcome occurs after the onset of the study, then the timing is prospective. Clinical trials are always prospective. Cohort and case-control studies may either be retrospective or prospective since the study may begin either before or after the health outcome has occurred. The timing of a study can have important implications for the quality of the data. Retrospective data are often based on personal recall, or on hospital or employment records, and are therefore more likely than prospective studies to involve measurement errors. Measurement errors frequently lead to information bias, which we discuss in a later chapter. Summary Timing answers the question: has the health outcome of interest already occurred before the study actually began? If the health outcome occurs before the study is initiated, the timing is retrospective. If the health outcome occurs after the onset of the study, the timing is prospective. Timing affects measurement error and information bias.

Clinical Trials
A clinical trial is an experimental study designed to compare the therapeutic or health benefits of two or more treatments. The major objective of a clinical trial is to test the efficacy of a preventive or therapeutic intervention. The long-range goal of a preventive trial is to prevent disease; the long-range goal of a therapeutic trial is to cure or control a disease. Examples of preventive trials include studies of vaccine efficacy, use of aspirin to prevent coronary heart disease, smoking cessation, diet modification, and exercise. Therapeutic trials are typically performed by pharmaceutical companies to test new drugs for treating disease. Key features of any clinical trial are randomization, blinding, ethical concerns, and the use of intention to treat analysis. Randomization is used to allocate subjects to treatment groups so that these groups are comparable on all

26 Chapter 3. How to Set Things Up? Study Designs factors except for exposure status. Blinding means that either the patient or the investigator is unaware of the treatment assigned. Single-blinding m eans either the patient or investigator are unaware of the treatment assignment and double blinding means that both the patient and the investigator are unaware of the treatment assignment. Blinding helps eliminate bias. The study must be ethical, treatments that may be harmful are not used. Stopping rules are planned that would end a trial early if it becomes clear that one of the treatments is superior. An intention-to-treat analysis requires that the investigators "analyze what they randomize", that is, analysis should be compared to the originally randomized treatment groups, even if study subjects switch treatments during the study period. Summary The major objective of a clinical trial is to test the efficacy of a preventive or therapeutic intervention. Key features of any clinical trial are: Randomization Ethical concerns Blinding Intention to treat analysis Clinical Trial Example A clinical trial involving 726 subjects conducted in 1993 compared standard insulin therapy with intensive insulin therapy involving more frequent insulin injections and blood glucose monitoring for the treatment of diabetes mellitus. The outcome studied was retinopathy resulting in blindness, defined as either present or absent for each patient. Subjects were randomized to treatment groups using a computerized random number generator. Double blinding could not be used in this clinical trial since both the patient and their physician would know which treatment group the patient was randomized. However, the individuals who graded the fundus photographs to determine the presence or absence of retinopathy were unaware of treatment-group assignments. The randomization resulted in the standard and intensive therapy groups having very similar distributions of baseline characteristics, such as age and sex. An intention-to-treat analysis compared the originally randomized treatment groups with regard to the occurrence of retinopathy. It was found that 24% of the 378 subjects on standard therapy developed retinopathy, whereas 6.7% of the 348 subjects on intensive therapy developed retinopathy. These data and more complicated analyses that controlled for several other important predictors indicated that intensive therapy had a much lower risk than standard therapy for retinopathy. Summary A clinical trial involving 726 subjects conducted in 1993 compared standard insulin therapy with intensive insulin therapy. The outcome studied was retinopathy resulting in blindness, defined as either present or absent for each patient.

A Pocket Guide to Epidemiology 27 24% of subjects on standard therapy developed retinopathy whereas 6.7% of subjects on intensive therapy developed retinopathy. Quiz (Q3.2) Fill in the Blanks; choices are Preventive or Therapeutic 1. ??? trials are conducted on individuals with a particular disease to assess a possible cure or control for the disease. For example, we may wish to assess to what extent, if at all, a new type of chemotherapy prolongs the life of children with acute lymphatic leukemia. 2. ??? trials can be conducted on either individuals or entire populations. An example is a study in which one community was assigned (at random) to receive sodium fluoride added to the water supply, while the other continued to receive water without supplementation. This study showed significant reductions in the development of tooth decay in the community receiving fluoride. For each of the following features, choose the option that applies to clinical trials: 3. The investigator 's role regarding exposure: . . ??? a. assign b. observe 4. Subject selection into groups: . . . . ??? a. self-selection b. randomization 5. Directionality: . . . . . ??? a. backwards b. forwards c. non-directional 6. Timing: . . . . . . ??? a. prospective b. retrospective c. either 7. Blinding: . . . . . . ??? a. single b. double c. either 8. Topic: . . . . . . ??? a. medication b. vaccine c. either 9. Analysis by: . . . . ??? a. original assignment b. actual experience

Observational Study Designs
There are three general categories of observational designs: Basic Designs: Cohort, Case-Control, Cross-Sectional Hybrid Designs: Nested Case-Control, Case-Cohort Incomplete Designs: Ecologic, Proportional

28 Chapter 3. How to Set Things Up? Study Designs

Cohort Studies
In 1948, a long-term observational study began in Framingham Massachusetts. Fifty-one hundred subjects without cardiovascular disease (CVD) were selected and examined, and information about potential risk factors for this disease was recorded. Subjects were then re-examined if possible every 2 years over the next 50 years. This classic study became known as the Framingham Heart Study and has been the source of much of our knowledge about risk factors for cardiovascular disease. The Framingham Heart study is an example of a prospective cohort study. A cohort design starts with subjects who do not have a health outcome of interest and are followed forward to determine health outcome status. A key feature of a cohort study is that subjects are grouped on the basis of their exposure characteristics prior to observing the health outcome, that is, the directionality of the study is always forward. A cohort study may be retrospective or prospective. The Framingham Heart study is an example of a prospective study since the study began before the health outcome occurred. Summary The Framingham Heart Study is a classic example of a cohort study. The cohort design is always a follow-up study with forward directionality. A cohort study can be prospective or retrospective. The Framingham study is a prospective cohort study because the study began before the health outcome occurred.

Advantages of a Cohort Study
The primary advantage of a cohort study is its forward directionality. The investigator can be reasonably sure that the hypothesized cause preceded the occurrence of disease. In a cohort study, disease status cannot influence the way subjects are selected, so a cohort study is free of certain selection biases that seriously limit other types of studies. A prospective cohort design is less prone than other observational study designs to obtaining incorrect information on important variables. Cohort studies can be used to study several diseases, since several health outcomes can be determined from follow-up. Cohort studies are also useful for examining rare exposures. Since the investigator selects subjects on the basis of exposure, he can ensure a sufficient number of exposed subjects. A retrospective cohort study can be relatively lowcost and quick. Occupational studies that are based on employment records and death certificates or insurance and worker 's comp records are an example.

A Pocket Guide to Epidemiology 29

Disadvantages of a Cohort Study
A prospective cohort study is often quite costly and time-consuming. A potential problem in any cohort study is the loss of subjects because of migration, lack of participation, withdrawal, and death. Such attrition of the cohort over the followup period can lead to biased results. A cohort design is statistically and practically inefficient for studying a rare disease with long latency because of the long follow-up time and the number of subjects required to identify a sufficient number of cases. However, a retrospective cohort study may find enough cases since the study events of interest have already occurred. Another problem in cohort studies is that the exposed may be followed more closely than the unexposed; if this happens, the outcome is more likely to be diagnosed in the exposed. This might create an appearance of exposure-disease relationship where none exists. Summary: Cohort Study +’s (advantages) and –’s (disadvantages) (+) Prospective cohort study: least prone to bias compared with other observational study designs. (+) Can address several diseases in the same study. (+) Retrospective cohort study: can be relatively low-cost and quick; frequently used in occupational studies. (-) Loss to follow-up is a potential source of bias (-) Prospective cohort study: quite costly and time-consuming; may not find enough cases if disease is rare. (-) If exposed are followed more closely than unexposed, the outcome is more likely to be diagnosed in exposed.

Example: Retrospective Cohort Study of Spontaneous Abortions
The relationship between adverse pregnancy outcomes and the use of video display terminals (VDT’s) became a public health concern in the 1980’s when adverse pregnancy outcomes were reported among several clusters of women who used VDT’s. A more comprehensive study of the effect of VDT’s was reported in the New England Journal of Medicine in 1991. This study, conducted by the National Institute for Occupational Safety and Health (NIOSH) used a retrospective cohort design to examine the hypothesis that electromagnetic energy produced by VDT’s might cause spontaneous abortions. In the NIOSH study, a cohort of female telephone operators who were employed between 1983 and 1986 was selected from employers’ personnel records at two telephone companies in eight southeastern states in the US. In this cohort, there were 882 women who had pregnancies

30 Chapter 3. How to Set Things Up? Study Designs that met the inclusion criteria for the study. Of these women, the pregnancy outcomes of 366 directory assistance operators who used VDT’s at work were compared with 516 general telephone operators who did not use VDT’s. The results of the study showed no excess risk of spontaneous abortion among women who used VDT’s during their first trimester of pregnancy. No doseresponse relation was found from the analysis of the women’s hours of VDT use per week either. Also, no excess risk was associated with VDT use when other relevant characteristics of the study subjects were taken into account. The investigators therefore concluded that the use of VDT’s and exposure to electromagnetic fields they produce were not associated with an increased risk of spontaneous abortion. Summary A 1991 study used a retrospective cohort design to examine the hypothesis that electromagnetic energy produced by VDT’s might cause spontaneous abortions. The pregnancy outcomes of 366 operators who used VDT’s at work were compared with 516 operators who did not use VDT’s The results of the study showed no excess risk of spontaneous abortion among women who used VDT’s.

Example: Prospective Cohort Study of Alzheimer’s Disease
Inflammatory activity in the brain is thought to contribute to the development of Alzheimer’s disease. This hypothesis suggests that long-term use of nonsteroidal anti-inflammatory drugs (NSAIDs),may reduce the risk of this disease. This hypothesis was investigated within the Rotterdam Study, a cohort study of the elderly that started in the Netherlands in 1990. At that time, 7,000 participants did not have Alzheimer’s disease. During eight years of follow-up, 293 of the participants developed the disease. To avoid information bias from measuring NSAIDs, the investigators used computerized pharmacy records instead of interview data to determine the total number of months during which participants had used NSAIDs after the study onset. Controlling for age, gender, and smoking status, the investigators found that the risk of Alzheimer’s for participants who had used NSAIDs for more than 24 months was significantly less than the risk of Alzheimer’s disease for participants who used NSAIDs for less than or equal to 24 months. The investigators concluded that long-term use of NSAIDs has a beneficial effect on the risk of Alzheimer’s disease. Summary The Rotterdam study examined the hypothesis that long-term use of nonsteriodal anti-inflammatory drugs (NSAIDs) may reduce the risk of Alzheimer’s disease. The study used a prospective cohort design that followed 7,000 participants without Alzheimer’s disease in 1990 over eight years. The risk of Alzheimer’s disease for subjects using NSAIDs for more than 24 months was significantly smaller than for subjects using NSAIDs less than or equal to 24 months.

A Pocket Guide to Epidemiology 31 Quiz (Q3.3) Fill in the Blanks. For each of the following features, choose the option that applies to cohort studies: 1. The investigator’s role regarding exposure: a. assign b. observe 2. Subject selection into groups: a. self-selection b. randomization 3. Directionality: a. backwards b. forwards c. non-directional 4. Timing: a. prospective b. retrospective c. either 5. Analysis by: a. original assignment b. actual experience ??? ??? ??? ??? ???

For each of the following characteristics (strengths or weaknesses) of a study, choose the type of cohort study with that characteristic; the choices are: Neither Prospective Cohort Retrospective Cohort Both 6. Less expensive: 7. Quicker: . . . . . . . . . . . . . . . . . . . . . . . . ??? ??? ??? ??? ??? ??? ???

8. More accurate exposure information:

9. Appropriate for studying rare exposures: . 10. Appropriate for studying rare diseases: 11. Problems with loss to follow-up: 12. Better for diseases with long latency:

Case-Control Studies
In case control studies, subjects are selected based on their disease status. The investigator first selects cases of a particular disease and then chooses controls from persons without the disease. Ideally, cases are selected from a clearly defined population, often called the source population, and controls are selected from the same population that yielded the cases. The prior exposure histories of cases and controls are then determined. Thus, in contrast to a cohort study, a case-control study works backwards from disease status to prior exposure status. While case-control studies are always backward in directionality, they can be either prospective or retrospective in timing. In addition to being both cheaper and quicker than cohort studies, casecontrol studies have other advantages:

32 Chapter 3. How to Set Things Up? Study Designs They are feasible for obtaining sufficient numbers of cases when studying chronic or other rare diseases or diseases with long latency periods. They tend to require a smaller sample size than other designs. They can evaluate the effect of a variety of different exposures. There are, nevertheless, several disadvantages of case-control studies: They do not allow several diseases to be evaluated, in contrast to cohort studies. They do not allow disease risk to be estimated directly because they work backwards from disease to exposure They are more susceptible to selection bias than other designs since the exposure has already occurred before cases and controls are selected. They are more susceptible to information bias than cohort studies because they are always backward in directionality. They are not efficient for studying rare exposures Summary Start with cases and non-cases of a disease or other health outcome and proceed backwards to determine prior exposure history. Popular primarily because cheaper and less time-consuming than cohort studies. Other advantages include providing sufficient numbers of cases for rare diseases with long latencies and allowing several exposures to be evaluated. Disadvantages include being susceptible to both selection and information bias, not allowing estimation of risk, not considering more than one disease, and not feasible for rare exposures.
Incident versus Prevalent Cases in a Case-Control Study? Cases can be chosen to be either incident or prevalent. Incident cases are new cases of a disease that develop over the time-period covered by the case-control study. When used in case-control studies, incident cases are typically obtained from an institutional or population-based disease registry, such as a cancer registry, or a health maintenance organization that continuously records new illnesses in a specified population. Prevalent cases are existing cases of a disease at a point in time. When used in casecontrol studies, prevalent cases are usually obtained from hospital or clinic records. An advantage of using of incident cases in case-control studies is that an exposuredisease relationship can be tied only to the development rather than the prognosis or duration of the disease. In contrast, for prevalent cases, the exposure may affect the prognosis or the duration of the illness. If prevalent cases were used, therefore, an estimate of the effect of exposure on disease development could be biased because of failure to include cases that died before case-selection.

Choosing Controls in a Case-Control Study
One must select a comparison or control group carefully when conducting a casecontrol study. The ideal control group should be representative of the population from which the cases are derived, typically called the source population. This ideal is often hard to achieve when choosing controls.

A Pocket Guide to Epidemiology 33 Two common types of controls are population-based controls and hospitalbased controls. In population-based case-control studies, controls are selected from the community. Methods used to select such controls include random telephone dialing, friend or neighborhood, and department of motor vehicle listings. An advantage of a population-based case-control study is that cases and controls come from the same source population, so they are similar in some way. A disadvantage is that it is difficult to obtain population lists and to identify and enroll subjects. Increasing use of unlisted numbers and answering machines increases non-response by potential controls. In a hospital-based case-control study, controls are selected from hospital patients with illnesses other than the disease of interest. Hospital controls are easily accessible and tend to be more cooperative than population-based controls. Hospital-based studies are much less expensive and time-consuming than population-based studies. But, hospital-based controls are not likely to be representative of the source population that produced the cases. Also, hospitalbased controls are ill and the exposure of interest may be a determinant of the control illness as well as the disease of interest. If so, a real association of the exposure with the disease of interest would likely be missed. Summary The ideal control group should be representative of the source population from which the cases are derived. Two common types of controls are population-based controls and hospitalbased controls. In population-based case-control studies, cases and controls come from the same source population. Hospital controls are easily accessible, tend to be cooperative, and are inexpensive. Hospital controls do not usually represent the source population but may represent an illness caused by the exposure.

Example: Case-Control Study of Reye’s Syndrome
Reye’s syndrome is a rare disease affecting the brain and liver that can result in delirium, coma, and death. It usually affects children, and typically occurs following a viral illness. To investigate whether aspirin is a determinant of Reye’s Syndrome, investigators in the nineteen seventies and nineteen eighties decided that using a clinical trial would not be ethical. Why might a clinical trial on aspirin use and Reye’s syndrome be unethical? (Choose one of the four choices below) A. Children are involved. B. Harmful consequences of the use of aspirin. C. Double blinding may be used. D. Clinical trials are never ethical. The answer is B, because of the potential harmful consequences of the use of aspirin. A cohort study was also considered inefficient:

34 Chapter 3. How to Set Things Up? Study Designs Why would a cohort study of aspirin and Reye’s syndrome be inefficient? Choose one of the four choices below) A. The outcome is rare (would require a lot of subjects). B. Requires at least 5 years of follow-up. C. The exposure is rare. D. Cohort studies are always inefficient The answer is A, because the outcome is so rare. Consequently, a case-control study was preferred, since such a study could be accomplished over a shorter period, provide a sufficient number of cases, yet require fewer subjects overall than a cohort study. The original investigation of Reye’s Syndrome that identified aspirin as a risk factor was a case-control study conducted in Michigan in 1979 and 1980. This study involved 25 cases and 46 controls. Controls were children who were absent from the same school, in a similar grade, had a similar time of preceding illness, had the same race, the same year of birth, and the same type of preceding illness. A larger 1982 study attempted to confirm or refute the earlier finding. Investigators used a statewide surveillance system to identify all cases with Reye’s syndrome in Ohio. This study thus used newly developed, or incident, cases. Population-based controls were selected by identifying and then sampling subjects in the statewide community who had experienced viral illnesses similar to those reported by the cases but had not developed Reye’s syndrome. Parents of both cases and controls were asked about their child’s use of medication during the illness. Another study published in 1987 selected cases from children admitted with Reye’s syndrome to any of a preselected group of tertiary care hospitals. Hospital-based controls were selected from children from these same hospitals who were admitted for a viral illness but did not develop Reye’s syndrome. Parents were interviewed to assess previous use of aspirin. As a result of this case-control research on the relationship between use of aspirin and Reye’s syndrome, health professionals recommended that aspirin not be used to treat symptoms of a viral illness in children. Subsequently, as the use of aspirin among children declined, so did the occurrence of Reye’s syndrome. Summary In the 1970s and 1980s, case-control studies were used to assess whether the use of aspirin for the treatment of viral illnesses in children was a determinant of Reye’s syndrome. These studies started with subjects with the disease (i.e., Reye’s syndrome) and similar subjects without the disease. Parents of both cases and controls were asked about their child’s use of medication over a comparable time period preceding the child’s first symptoms of Reye’s syndrome.

A Pocket Guide to Epidemiology 35 Health professionals recommended that aspirin not be used to treat symptoms of viral illnesses in children. As the use of aspirin among children declined, so did the occurrence of Reye’s syndrome.

Example: Case-Control Study on Creutzfeldt-Jakob Disease
Creutzfeldt-Jakob disease (CJD) is a rare disease characterized by rapidly progressive dementia. In the 1990 's, a new variant of CJD in humans was discovered in Europe following an epidemic in cattle of mad cow disease, the animal form of CJD. Subsequently, the European Union organized a study to investigate whether a diet containing animal products is a risk factor for CJD. Because CJD is a very rare disease with a long latency period, the investigators chose a case-control study design. They collected data on 405 cases of CJD that had occurred in the European Union. An equal number of control participants were recruited from the hospitals where the patients with CJD had been diagnosed. Due to the mental deterioration of patients from the disease, diet information on cases had to be collected by interviewing one of the cases ' next of kin. How do you think the investigators collected diet information on control subjects? Even though the control participants were perfectly capable of giving information about their diets themselves, the investigators interviewed one of the control participants ' next of kin instead. This way, they tried to avoid information bias by keeping the quality of the data on diet similar for both cases and controls. Remember that one of the advantages of a case-control study is the opportunity to evaluate the effect of a variety of different exposures. In this study, the investigators examined separately whether consumption of sausage, raw meat, raw fish, animal blood products, milk, cheese, as well as other specified animal products, increased the risk of CJD. None of these food products significantly increased the risk of CJD, so, the investigators concluded that it is unlikely that CJD is transmitted from animals to man via animal products. Quiz (Q3.4) For each of the following, choose the option that applies to case-control studies: 1. The investigator 's role regarding exposure: a. assign b. observe 2. Subject selection into groups: a. self-selection b. randomization 3. Directionality: a. backwards b. forwards c. non-directional 4. Timing: a. prospective b. retrospective c. either 5. Analysis by: a. original assignment b. actual experience ??? ??? ??? ??? ???

36 Chapter 3. How to Set Things Up? Study Designs For each of the following characteristics (strengths or weaknesses) of a study, choose the type of study with that characteristic. The choices are: Prospective cohort Case-control 6. Less expensive: 7. Quicker: . . . . . . . . . . . . . . . . . . . . . . . . . . . ??? ??? ??? ??? ??? ??? ??? ???

8. More accurate exposure information:

9. Appropriate for studying rare exposures: . 10. Appropriate for studying rare diseases: 11. Can study multiple outcomes: 12. Requires a smaller sample size: . 13. Can estimate risk: .

Determine whether each of the following statements is true or false: 14. Ideally, controls should be chosen from the same population that gave rise to the cases. . . ??? ???

15. Ideally, controls should be selected from hospitalized patients

16. Population-based controls include only neighbors and persons identified by calling random telephone numbers. . ???

Cross-Sectional Studies
In a cross-sectional study, subjects are sampled at a fixed point or within a short period of time. All participating subjects are examined, observed, and questioned about their disease status, their current or past exposures, and other relevant variables. A cross-sectional study provides a snapshot of the health experience of a population at a specified time and is therefore often used to describe patterns of disease occurrence. A cross-sectional sample is usually more representative of the general population being studied than are other study designs. A cross-sectional study is a convenient and inexpensive way to look at the relationships among several exposures and several diseases. If the disease of interest is relatively common and has long duration, a cross-sectional study can provide sufficient numbers of cases to be useful for generating hypotheses about exposure-disease relationships. Other more expensive kinds of studies, particularly cohort and clinical trials, are used to test such hypotheses. There are some disadvantages to cross-sectional studies. For example, such a

A Pocket Guide to Epidemiology 37 study can identify only existing or prevalent cases at a given time, rather than new or incident cases over a follow-up time period. Therefore, a cross-sectional study cannot establish whether the exposure preceded the disease or whether the disease influenced the exposure. Because only existing cases are allowed, a cross-sectional study includes only cases that survive long enough to be available for study. This could lead to a misleading conclusion about an exposure-disease relationship since non-survivors are excluded (see note at the end of this chapter on this issue). Short-duration diseases, such as the common cold or influenza, especially those that occur during a particular season, may be under-represented by a crosssectional study that looks at the presence of such a disease at a point in time. Summary: Cross-Sectional Studies Subjects are sampled at a fixed point or short period of time: a snapshot. Advantages Convenient and inexpensive. Can consider several exposures and several diseases. Can generate hypotheses. Usually represents the general population. Disadvantages Cannot establish whether the exposure preceded disease or disease influence exposure. Possible bias since only survivors are available for study. May under-represent diseases with short duration.
How Can Bias Occur from Survivors in a Cross-sectional Study? In a cross-sectional study, bias can result because only cases that survive long enough are available for such a study. To illustrate this point, suppose that everyone with a certain disease who does not do strenuous physical exercise regularly dies very quickly. Suppose, also, that those who have the disease but do strenuous physical exercise regularly survive for several years. Now consider a cross-sectional study to assess whether regular strenuous physical activity is associated with the disease. Since this type of study would contain only survivors, we would likely find a low proportion of cases among persons not doing strenuous physical exercise. In contrast, we would likely find a relatively higher proportion of cases among persons who do strenuous physical exercise. This would suggest that doing strenuous physical exercise is harmful for the disease, even if, in fact, it were protective.

Example of a Cross-Sectional Study – Peripheral Vascular Disease, Scotland A 1991 study examined a sample of 5000 Scottish men for the presence of peripheral vascular disease (PVD). Other characteristics, including whether or not a subject ever smoked, were also determined for each subject during the exam. This was a cross-sectional study since all study subjects were selected and observed at one point in time. Even though physical exams were performed,

38 Chapter 3. How to Set Things Up? Study Designs the study cost and time was much less than that required if disease-free subjects were followed over time to determine future PVD status. The sample was representative of the Scottish male population. The study found that 1.3 percent of 1727 ever-smokers had PVD whereas only 0.6 percent of 1299 never-smokers had PVD. Dividing .013 by .006, we see that ever-smokers were 2.2 times more likely to have PVD than never-smokers. These results suggested that smoking may contribute to developing PVD. Yet, the results are just a snapshot of subjects at a point in time, 1991. Subjects without PVD have not been followed over time. So, how do we know from this snapshot whether PVD leads to smoking or smoking leads to PVD? This illustrates one of the problems with cross-sectional studies - they are always non-directional. Also, persons who died from PVD prior to the time that subjects were selected are not allowed in the study. Therefore, the study results may be biased because only PVD survivors are being counted. Summary An example of a cross-sectional study is a 1991 study of peripheral vascular disease (PVD) in Scotland. Results show that ever-smokers are 2.2 times more likely to have PVD than never-smokers. This study was much cheaper and quicker then a cohort study. Cannot determine whether PVD leads to smoking or smoking leads to PVD. The study results may be biased because only PVD survivors are considered. Quiz (Q3.5) For each of the following features, choose the option that applies to cross-sectional studies: 1. The investigator 's role regarding exposure: a. assign b. observe 2. Subject selection into groups: . . a. self-selection b. randomization 3. Directionality: . . . a. backwards b. forwards 4. Timing: . . . . a. prospective b. retrospective . . . . . ??? ??? . ??? c. non-directional . ??? c. either

Determine whether each of the following statements is true or false: 5. Cross-sectional studies are better suited to generating hypotheses about exposure-disease relationships than to testing such relationships. ??? 6. Because exposure and disease are assessed at the same time, cross-sectional studies are not subject to survival bias. . . . ??? 7. Because exposure and disease are assessed at the same time, cross-sectional studies may not be able to establish that exposure preceded onset of the disease process. . . . . . . ??? 8. Cross-sectional studies can examine multiple exposures and multiple diseases. . . . . . . . ???

A Pocket Guide to Epidemiology 39

Hybrid Designs
Hybrid designs combine the elements of at least two basic designs, or extend the strategy of one basic design through repetition. Two popular hybrid designs are the case-cohort study and the nested case-control study. Both these designs combine elements of a cohort and case-control study. Another more recently developed hybrid design is called the case-crossover design.

Incomplete Designs
Incomplete designs are studies in which information is missing on one or more relevant factors. An ecologic study is an incomplete design for which the unit of analysis is a group, often defined geographically, such as a census tract, a state, or a country. A proportional morbidity or proportional mortality study only includes observations on cases but lacks information about the candidate population at risk for developing the health outcome. If the design involves incident cases, the study is a proportional morbidity study. If deaths are used, the study is a proportional mortality study. See Lesson Page 3-3 in the ActivEpi CD ROM for further details on Hybrid and Incomplete Designs

References
Reference for Diabetes Research Group Study (Clinical Trial) The Diabetes Control and Complications Trial Research Group. The effect of intensive treatment of diabetes on the development and progression of long-term complications in insulin-dependent diabetes mellitus. N Engl J Med 1993; 329(14):977-86. Reference for Framingham Heart Study (Prospective Cohort Study) Feinleib M, The Framingham study: sample selection, follow-up, and methods of analysis, in National Cancer Institute Monograph, No. 67, Greenwald P (editor), US Department of Health and Human Services, 1985. Dorgan JF, Brown C, Barrett M, et al. Physical activity and risk of breast cancer in the Framingham Heart Study, Am J Epidemiol 1994;139(7): 662-9. Margolis JR, Gillum RF, Feinleib M, et al. Community surveillance for coronary heart disease: the Framingham Cardiovascular Disease Survey. Methods and preliminary results. Am J Epidemiol 1974;100(6):425-36. Reference for VDT use and Spontaneous Abortion (Retrospective Cohort Study) Schnorr TM, Grajewski BA, Hornung RW, et al. Video display terminals and the risk of spontaneous abortion. N Engl J Med 1991;324(11):727-33. Reference for Nonsteroidal anti-inflammatory drugs and Alzheimer’s disease (Prospective Cohort Study) in t ' Veld BA, Ruitenberg A, Hofman A, et al. Nonsteroidal antiinflammatory drugs and the risk of Alzheimer 's disease. N Engl J Med 2001;345(21):1515-21. References for Reye’s Syndrome (Case-Control Studies) Waldman RJ, Hall WN, McGee H, Van Amburg G. Aspirin as a risk factor in Reye 's syndrome. JAMA 1982;247(22):3089-94. Halpin TJ, Holtzhauer FJ, Campbell RJ, et al. Reye 's syndrome and medication use. JAMA 1982;248(6):687-91. Daniels SR, Greenberg RS, Ibrahim MA. Scientific uncertainties in the studies of salicylate use and Reye 's syndrome. JAMA 1983;249(10):1311-6. Hurwitz ES, Barrett MJ, Bregman D, et al. Public Health Service study of Reye 's syndrome

40 Chapter 3. How to Set Things Up? Study Designs and medications. Report of the main study. JAMA 1987;257(14):1905-11. Forsyth BW, Horwitz RI, Acampora D, et al. New epidemiologic evidence confirming that bias does not explain the aspirin/Reye 's syndrome association. JAMA 1989;261(17):2517-24 References for Creutzfeldt-Jakob Disease (Case-Control Studies) van Duijn CM, Delasnerie-Laupretre N, Masullo C, et al. Case-control study of risk factors of Creutzfeldt-Jakob disease in Europe during 1993-95. European Union (EU) Collaborative Study Group of Creutzfeldt-Jakob disease (CJD). Lancet 1998;351(9109):1081-5. Will RG, Ironside JW, Zeidler M, et al. A new variant of Creutzfeldt-Jakob disease in the UK. Lancet 1996;347(9006):921-5. General epidemiologic design Checkoway H, Pearce N, Dement JM. Design and conduct of occupational epidemiology studies: II. Analysis of cohort data. Am J Ind Med 1989;(15(4):375-94. Greenberg RS, Daniels SR, Flanders WD, et al. Medical Epidemiology (3rd Ed). Lange Medical Books, New York, 2001. Kleinbaum DG, Kupper LL, Morgenstern H. Epidemiologic Research: Principles and Quantitative Methods. John Wiley and Sons Publishers, New York, 1982. Example of Cross-Sectional Studies Smith WCS, Woodward M, Tunstall-Pedoe H. Intermittent claudication in Scotland, in Epidemiology of Peripheral Vascular Disease. (ed FGR Fowkes.), Springer-Verlag, Berlin, 1991 Hybrid designs Coates RJ, Weiss NS, Daling JR, et al. Cancer risk in relation to serum copper levels. Cancer Res 1989;49(15): 4353-6. Linn JT, Wang LY, Wang JT, et al. A nested case-control study on the association between Helicobacter pylori infection and gastric cancer risk in a cohort of 9775 men in Taiwan. Anticancer Res 1995;15:603-6. Maclure M, Mittleman MA. Should we use a case-crossover design? Annu Rev Public Health 2000;21:193-221. Maclure M. The case-crossover design: a method for studying transient effects on the risk of acute events. Am J Epidemiol 1991;133(2):144-53. Redelmeier DA, Tibshirani RJ. Association between cellular-telephone calls and motor vehicle collisions. N Eng J Med 1997;336(7):453-8. Ecologic CDC. Summary of notifiable diseases in the United States. MMWR Morb Mortal Wkly Rep 1990;38(54):1-59. Morgenstern H. Ecologic studies in epidemiology: concepts, principles, and methods. Ann Rev Public Health 1995;16:61-81. Proportional Mortality Mancuso TF, Stewart A, Kneale G. Radiation exposures of Hanford workers dying from cancer and other causes. Health Phys 1977;33:369-85. Causation and Meta Analysis Blalock HM, Causal Inferences in Nonexperimental Research, Chapter 1, Norton Publishing, 1964. Chalmers I, Altman DG (eds.), Systematic Reviews, BMJ Publishing Group, London, 1995. Chalmers TC. Problems induced by meta-analyses. Stat Med 1991;10(6):971-80. Hill AB, Principles of Medical Statistics, 9th Edition, Chapter 24, Oxford University Press, 1971. Lipsey MW, Wilson DB. Practical meta-analysis. Applied Social Research Methods Series; Vol. 49. Sage Publications, Inc., Thousand Oaks, CA: 2001. Mosteller F, Colditz GA. Understanding research synthesis (meta-analysis). Annu Rev

A Pocket Guide to Epidemiology 41
Public Health 1996;17:1-23. Petitti DB, Meta-analysis Decision Analysis and Cost-Effectiveness Analysis; Methods for Quantitative Synthesis in Medicine, Oxford University Press, 1994. Popper KR, The Logic of Scientific Discovery, Harper and Row Publishers, 1968. Rothman KJ. Causes. Am J Epidemiol 1976;104(6): 587-92. Susser M, Causal Thinking in the Health Sciences, Oxford University Press, 1973. U.S. Department of Health, Education, and Welfare, Smoking and Health, PHS Publ. No. 1103, Government Printing, Washington DC, 1964. Weiss NS. Inferring causal relationships: elaboration of the criterion of dose-response. Am J Epidemiol 1981;113(5):487-90.

Answers to Quizzes
Q3.1 1. Experimental 2. Observational 3. Observational 4. Experimental 5. Experimental 6. Observational Q3.2 1. Therapeutic 2. Preventive 3. a 4. b 5. b 6. a 7. c 8. c 9. a Q3.3 1. b 2. a 3. b 4. c 5. b 6. Retrospective 7. Retrospective 8. Prospective 9. Both 10. Neither 11. Both 12. Retrospective Q3.4 1. b 2. a 3. a 4. c 5. b 6. 7. 8. 9. 10. 11. 12. 13. 14. case-control case-control prospective cohort prospective cohort case-control prospective cohort case-control prospective cohort T – If controls are chosen from a different population from which the cases came, there may be selection bias. 15. F – Hospital controls have an illness; such controls are typically not representative of the community from which the cases came. 16. F – Population-based controls can be obtained from random dialing of telephone numbers in the community from which the cases are derived. There is no guarantee that neighbors of cases will be chosen. Q3.5 1. b 2. a 3. c 4. b 5. T 6. F 7. T – A cross-sectional study includes only cases that survive long enough to be available for study. This could lead to a misleading conclusion about an exposure-disease relationship since non-survivors are excluded. 8. T

CHAPTER

4

HOW OFTEN DOES IT HAPPEN? DISEASE FREQUENCY
In epidemiologic studies, we use a measure of disease frequency to determine how often the disease or other health outcome of interest occurs in various subgroups of interest. We describe two basic types of measures of disease frequency in this chapter, namely, measures of incidence and measures of prevalence. The choice typically depends on the study design being used and the goal of the study.

Incidence versus Prevalence
There are two general types of measures of disease frequency, incidence (I) and prevalence (P). Incidence measures new cases of a disease that develop over a period of time. Prevalence measures existing cases of a disease at a particular point in time or over a period of time. To illustrate how incidence and prevalence differ, we consider our experience with AIDS. The number of annual incident cases of AIDS in gay men decreased in the US from the mid-1980s to the late 1990s. This has resulted primarily both from recent anti-retroviral treatment approaches and from prevention strategies for reducing high-risk sexual behavior. In contrast, the annual prevalent cases of AIDS in gay men has greatly increased in the US during the same period because recent treatment approaches for AIDS have been successful in prolonging life of persons with the HIV virus and/or AIDS.

Prevalence can also be viewed as describing a pool of disease in a population, whereas incidence describes the input flow of new cases into the pool, and fatality and recovery reflects the output flow from the pool. Incidence measures are useful for identifying risk factors and assessing disease etiology. Typically, incidence measures are estimated from clinical trials and from cohort studies, which involve the follow-up of subjects over time.

44 Chapter 4. How Often does it Happen? Disease Frequency Prevalence measures are not as useful as incidence measures for assessing etiology because prevalence does not consider persons who die from the disease before the prevalence study begins. Typically, prevalence measures are estimated from cross-sectional studies and from case-control studies that use prevalent rather than incident cases. Since the number of prevalent cases indicates demand for health care, prevalence measures are most useful in the planning of health services. Summary Incidence concerns new cases of disease or other health outcome over a period of follow-up. Prevalence concerns existing cases of a disease at a point in time. Incidence measures are useful for identifying risk factors and assessing disease etiology Prevalence measures are most useful in the planning of health services
Mortality Might Be Used Instead of Disease Incidence We discuss incidence and prevalence in terms of new or existing cases of a disease, whether or not these cases eventually die or not during or after the period of study. There are many situations, however, when the use of strictly mortality information is also worthwhile. Mortality measures are an important tool for epidemiologic surveillance. Today such surveillance programs have been applied to monitor the occurrence of a wide variety of health events, including deaths, in large populations. Mortality statistics are also convenient for evaluating etiologic hypotheses, especially when incidence data are not available. In particular, for diseases with a low rate of cure or recovery, such as lung cancer, mortality measures give a reasonable approximation to incidence measures. Use of mortality information for any of the above purposes has several pragmatic advantages: Mortality data are widely collected and virtually complete since registration of deaths is compulsory in most industrialized countries and few deaths are not reported. Mortality data are defined using standardized nomenclature. In particular, the International Classification of Diseases (ICD) is used to promote uniformity in reporting causes of death. Recording of mortality data is relatively inexpensive.

House Guests Example
Suppose guests arrive at your house at the rate of two per day and stay exactly five days. How many people will be in your house after a week? Let 's see what happens day by day. On the first day, two guests arrive and none depart, so there are 2 guests in your house at the end of the first day. On the second day two more guests arrive, and none depart, so there are now 4 guests in your house after 2 days. Similarly, there are 6 guests after 3 days, 8 after 4 days and 10 guests in your house after five days, with no guests departing up to this point. But, on the sixth day, two new guests arrive, but the two guests that came on day 1, having been there for five days, now depart, leaving you again with 10 guests in the house. At the end of the seventh day, there will still be 10 guests in the house, which answers the question raised at the start of all this.

A Pocket Guide to Epidemiology 45 This scenario illustrates the fundamental difference between incidence and prevalence. In the example, after 5 days, a "steady state" is reached at which point there are 10 houseguests as long as the arrival rate is 2 per day. This steady state of 10 houseguests is a prevalence, which describes the existing count of guests, at any point in time after a steady state has been reached. The arrival rate of 2 guests per day is an incidence, which describes how quickly new guests are arriving. The duration of five days that guests stay in your house is the information needed to link the incidence to the prevalence. Prevalence can be linked to incidence with the following formula: P = I x D In our example, P is the number of guests in the house on any day after day five, I is the arrival rate of 2 guests per day, and D is the duration of 5 days for each guest. The formula works in this example since 2 times 5 equals 10. We can see from this formula that for a given incidence, the prevalence will increase or decrease as the duration increases or decreases. For example, if guests stayed for 8 days rather than 5 days, with the same arrival rate, the number of guests at the house at steady state would be 2 times 8, which equals 16, rather than 10. For a given duration, the prevalence will increase or decrease as the incidence increases or decreases. Thus, if the guests arrive at the rate of only 1 guest per day rather than 2, and stay 8 days, the prevalence will be 1 times 8, which equals 8, instead of 16. Summary A scenario involving houseguests who arrive at 2 per day and stay five days illustrates the fundamental difference between incidence and prevalence. A steady state of 10 houseguests illustrates prevalence, which describes the existing count of guests at any point in time after steady state is reached. The arrival of 2 guests per day illustrates incidence, which describes how quickly new guests are arriving. The duration of 5 days is the information needed to link how incidence leads to prevalence Prevalence is obtained as the product of incidence and duration (P = I x D)
The Relationship between Prevalence and Incidence In the example involving "house guests", the formula P ( I D) was used to demonstrate that the steady state number of guests in the house after 7 days was equal to the product of the number of guests arriving each day times the duration that each guest stayed in the house. The terms P, I, and D in this formula represent the concepts of prevalence, incidence and duration, respectively, but, as used in the example, they each do not strictly conform to the epidemiologic definitions of these terms. As described in later activities in this chapter on measures of disease frequency, the strict definitions of prevalence and incidence require denominators, whereas the "house guest" scenario described here makes use only of numerator information. Specifically, prevalence is estimated using the formula:

P

C N

and incidence uses one of the following two possible formulas depending on whether risk or rate is the incidence measure chosen:

46 Chapter 4. How Often does it Happen? Disease Frequency

CI

I N

or

IR

I PT

In the above formulae, P, C, and N denote the prevalence, number of existing cases, and steady state population-size, respectively. Also, CI denotes cumulative incidence, which estimates risk, I denotes the number of new (incident cases), and N denotes the size of a disease-free cohort followed over the entire study period. Further, IR stands for incidence rate, and PT for accumulated person-time information. All these formulae are described and illustrated in later activities. The important point being made here is that all three of the above formulae have denominators, which were not used in the houseguest example, but are required for computing prevalence and incidence in epidemiology. The term D in the formula at the top of this page was used in the houseguest example to define the duration of stay that was assumed for each houseguest. In the epidemiologic use of this formula, D actually denotes the average duration of illness for all subjects in the population under study, rather than being assumed to be the same for each person in the population. Nevertheless, using the stricter epidemiologic definitions of prevalence and incidence measures and using average duration, the above formula that relates prevalence to incidence and duration still holds, provided the population is in steady state and the disease is rare. By steady state, we mean that even though the population may be dynamic, the number of persons who enter and leave the population for whatever reasons are essentially equal over the study period, so that the population does not change. If the disease is not rare, a modified formula relating prevalence to incidence is required instead, namely:

P

I (I

D D) 1

Quiz (Q4.1) For each of the following scenarios, determine whether it is more closely related to incidence or to prevalence. 1. Number of campers who developed gastroenteritis within a few days after eating potato salad at the dining hall? ??? 2. Number of persons who reported having with diabetes as part of the National Health Interview Survey? ??? 3. Occurrence of acute myocardial infarction (heart attack) among participants during the first 10 years of follow-up of the Framingham Study? ??? 4. Number of persons who died and whose deaths were attributed to Hurricane Floyd in North Carolina in 1999? ??? 5. Number of children who have immunity to measles, either because they had the disease or because they received the vaccine? ??? Suppose a surveillance system was able to accurately and completely capture all new occurrences of disease in a community. Suppose also that a survey was conducted on July 1 that asked every member of that community whether they currently had that disease. For each of the following conditions, determine whether incidence (per 1,000 persons per year) or prevalence (per 1,000 persons on July 1) is likely to be higher.

A Pocket Guide to Epidemiology 47 6. Rabies (occurs rarely and has a short duration, e.g., death within one week)? ??? 7. Multiple sclerosis (rare occurrence, long duration [many years])? ??? 8. Influenza (common but winter-seasonal occurrence, short duration)? ??? 9. Poison ivy dermatitis (common spring/summer/fall occurrence, 2-week duration)? ??? 10. High blood pressure (not uncommon occurrence, lifelong duration)? ???

Risk
The term risk is commonly used in everyday life to describe the likelihood, or probability, that some event of interest will occur. We may wonder, for example, what is the risk that the stock market will crash or that we will be involved in a serious auto collision? We may worry about our risk for developing an undesirable health condition, such as a life-threatening illness, even our risk for dying. In epidemiology, risk is the probability that an individual with certain characteristics, say, age, race, sex, and smoking status, will develop or die from a disease, or even more generally, will experience a health status change of interest over a specified follow-up period. When the health outcome is a disease, this definition assumes that the individual does not have the disease at the start of follow-up and does not die from any other cause during follow-up. Because risk is a probability, it is a number between 0 and l, or, correspondingly, a percentage. When describing risk, it is necessary to specify a period of follow-up, called the risk period. For example, to describe the risk that a 45 year-old male will develop prostate cancer, we must state the risk period, say, 10 years of follow-up, over which we want to predict this risk. If the risk period were, for example, 20 years instead of 10 years, we would expect our estimate of risk to be larger than the 10-year risk since more time is being allowed for the disease to develop. Study Questions (Q4.2) 1. What is the meaning of the following statement? The 10-year risk that a 45year-old male will develop prostate cancer is 5%? (State your answer in probability terms and be as specific as you can in terms of the assumptions required.) 2. Will the 5-year risk for the same person described in the previous question be larger or smaller than the 10-year risk? Explain briefly. Summary Risk is the probability than an individual will develop or die from a given disease or, more generally, will experience a health status change over a specified follow-up period. Risk assumes that the individual does not have the disease at the start of the follow-up and does not die from any other cause during the follow-up. Risk must be some value between 0 and 1, or correspondingly, a percentage When describing risk, it is necessary to give the follow-up period over which the risk is to be predicted.

48 Chapter 4. How Often does it Happen? Disease Frequency
Confusing Risk with Rate The term rate has often been used incorrectly to describe a measure of risk. For example, the term attack rate is frequently used in studies of outbreaks to describe an estimate of the probability of developing an infectious illness, when in fact, an estimate of risk is computed. Also, the term death rate has been confused with death risk in mortality studies. In particular, the term case-fatality rate has often been misused to describe the proportion of cases that die, i.e., such a proportion is actually estimating a risk. The terms risk and rate have very different meanings, as described in other activities in this chapter. Ideally, the correct term should be applied to the actual measure being used. This does not always happen in the publication of epidemiologic findings. Consequently, when reading the epidemiologic literature, one should be careful to determine the actual measure being reported.

Cumulative Incidence
The most common way to estimate risk is to divide the number of newly detected cases that develop during follow-up by the number of disease-free subjects available at the start of follow-up. Such an estimate is often called cumulative incidence or CI. When describing cumulative incidence, it is necessary to give the follow-up period over which the risk is estimated.

CI

I N

# of new cases during follow - up # of disease - free subjects at start of follow - up

Technically speaking, cumulative incidence is not equivalent to individual risk, but rather is an estimate of individual risk computed from either an entire population or a sample of a population. However, we often use the terms risk and cumulative incidence interchangeably, as we do throughout this course. We usually put a hat (“^”) over the CI when the estimate of cumulative incidence is based on a sample; we leave off the hat if we have data for the entire population. CI “hat” The cumulative incidence formula, with or without a "hat", is always a proportion, so its values can vary from 0 to 1. If the cumulative incidence is high, as in an outbreak, the CI is sometimes expressed as a percent. As a simple example, suppose we followed 1000 men age 45 and found that 50 developed prostate cancer within 10 years of follow-up and that no subject was lost to follow-up or withdrew from the study. Then our estimate of simple cumulative incidence is 50 over 1000, or 0.05, or, 5 %.

ˆ CI

ˆ CI

I N

50 .05 5% 1000

In other words, the 10-year risk, technically the cumulative incidence for a 45 year-old male is estimated to be 5%. The formula we have given for computing risk is often referred to as Simple Cumulative Incidence because it is a simple proportion that assumes a fixed cohort. Nevertheless, the use of simple cumulative incidence is not always appropriate in all kinds of follow-up studies. Problems with simple cumulative incidence and methods for dealing with such problems are discussed in activities to follow.

A Pocket Guide to Epidemiology 49 Summary Cumulative incidence (CI) is a population-based estimate of individual risk Cumulative incidence is always a proportion When describing cumulative incidence, it is necessary to give the follow-up period over which the risk is estimated. The formula for simple cumulative incidence is CI=I/N, where I denotes the number of new cases of disease that develop over the follow-up period and N denotes the size of the disease-free population at the start of follow-up. The terms cumulative incidence and risk are used interchangeably here, even though technically, they are different.

Shifting the Cohort
The formula for simple cumulative incidence implicitly assumes that the cohort is "fixed" in the sense that no entries into the cohort are allowed during the follow-up period. What we should do if we do allow new entries into the cohort? For example, in the Sydney Beach Users study described in Chapter 2, subjects were selected from 12 popular Sydney beaches over 41 sampling days throughout the summer months of 1989-90. Subjects could progressively enter the cohort on different days during the summer, after which self-reported exposure and disease information were obtained one week later. To illustrate, consider six subjects shown to the right. Each subject is followed for the required 7 days. Subjects 1 and 5 (going from the bottom individual to the top individual) are the only subjects who reported becoming ill. We can restructure these data by shifting the line of follow-up for each person to the left margin so that the horizontal time axis now reflects days of observation from the start of observation for each subject, rather than the actual calendar days at which the observations occurred. This conforms to the follow-up of a fixed cohort, for which the cumulative incidence is estimated to be 2/6 or onethird.

We often have a cohort that allows subjects to progressively enter the study at different calendar times. We can restructure the cohort to be fixed by shifting the data for each subject to reflect the time of observation since initial entry into the study rather than calendar time. We can then use the simple cumulative incidence

50 Chapter 4. How Often does it Happen? Disease Frequency formula to estimate risk. Study Questions (Q4.3) Suppose after shifting the cohort, one subject remained disease-free during 4 years of follow-up whereas another subject in the cohort remained disease-free but was only followed for 2 years. 1. Do we have to assume that subjects who became cases were followed for the same amount of time as subjects who remained disease-free? 2. Is there a problem with computing the cumulative incidence that includes both these subjects in the denominator of the CI formula? 3. After we have shifted the cohort, do we have to assume that ALL subjects, including those who became cases, were followed for the same amount of time in order to compute cumulative incidence (CI)? Summary If subjects progressively enter the study at different calendar times, the data can be shifted to reflect the time of observation since initial entry. Simple cumulative incidence can be used to estimate risk for a shifted cohort. After shifting the cohort, we can compute cumulative incidence provided all subjects who remained disease-free throughout follow-up are followed for the entire length of follow-up.

Problems with Simple Cumulative Incidence
There are several potential problems with assuming a fixed cohort when using the formula for simple cumulative incidence to estimate risk. One problem occurs because the size of a fixed cohort is likely to be reduced during the follow-up period as a result of deaths or other sources of attrition such as loss to followup or withdrawal from the study. We don 't know whether a subject lost during follow-up developed the disease of interest. Another problem arises if the population studied is a dynamic population rather than a fixed cohort. A fixed cohort is a group of subjects identified at some point in time and followed for a given period for detection of new cases. The cohort is "fixed" in the sense that no entries are permitted into the study after the onset of follow-up, although subsequent losses of subjects may occur for various reasons such as withdrawal, migration, and death. But, a dynamic population is continually changing, allowing for both the addition of new members and the loss of previously entered members during the follow-up period. The denominator in the simple cumulative incidence formula does not reflect the continually changing population size of a dynamic population. And the numerator in the simple cumulative incidence formula does not count new cases that may arise from those persons who entered a dynamic population after the beginning of follow-up. Another difficulty for either a fixed or dynamic cohort is that subjects may be followed for different periods of time so that a cumulative incidence estimate will not make use of differing follow-up periods. This problem can occur when subjects are lost to follow-up or withdraw from the study. It could also occur if subjects enter the study after the study start and are disease-free until the study

A Pocket Guide to Epidemiology 51 ends, or if the follow-up time at which a subject develops the disease varies for different subjects. To illustrate these problems, let 's consider a hypothetical example involving 12 initially disease-free subjects who are followed over a 5- year period from 1990 to 1995. An X denotes the time at which a subject was diagnosed with the disease and a circle (O) denotes the time of death that could be due to the disease (circle with an X inside) or due to another cause (circle without an X). Those subjects that have no X or circle on their time line either withdrew from the study, or were lost to follow-up, or were followed until the end of the study without the developing the disease. The value to the right of each subject 's time line denotes that subject 's follow-up time period until either the disease was diagnosed, the subject withdrew or was lost to follow-up, or until the study ended. Based on this information, answer the following questions: Study Questions (Q4.4) The questions below refer to the figure above: 1. What type of cohort is being studied, fixed or dynamic? 2a. Which of these subjects was diagnosed with the disease? Subject 2 Subject 3 Subject 5 Subject 7 2b. Which of these subjects was lost or withdrawn? Subject 2 Subject 3 Subject 5 Subject 7 2c. Which of these subjects died with disease? Subject 3 Subject 5 Subject 7 Subject 9 2d. Which of these subjects died without the disease? Subject 3 Subject 5 Subject 7 Subject 9 2e. Which one was without the disease and alive at the end? Subject 3 Subject 5 Subject 7 Subject 9 Shifted Cohort

3. If we could shift the cohort, what is your estimate of simple cumulative incidence of disease diagnosis in percent?

52 Chapter 4. How Often does it Happen? Disease Frequency 4. What is your estimate of simple cumulative incidence of death in percent with no decimal places? 5. Using the “unshifted” graph from the previous page, Subjects 5, 8, 10 and 12 have which of the following in common: A. Same amount of observed follow-up time B. Entered study at same calendar time C. Withdrew from the study D. Did not develop disease during follow-up Computing simple cumulative incidence for the previously shown data is a problem because … 6a. Not all subjects developed the disease Yes No 6b. Not all subjects died Yes No 6c. The cohort is dynamic Yes No Yes No 6d. Some subjects died from another cause 6e. Some subjects were lost or withdrew Yes No 6f. Some subjects developed disease at different follow-up times Yes No 6g. Subjects not developing the disease had different follow-up times Yes No Summary There are problems with assuming a fixed cohort when using the formula for simple cumulative incidence to estimate risk. If there is attrition of a fixed cohort, we will not know whether a subject lost during follow-up developed the disease. For a dynamic cohort, the denominator in the simple cumulative incidence formula does not reflect the continually changing population size Simple cumulative incidence does not allow subjects to be followed for different periods of time. Quiz (Q4.5) After the second game of the college football season, 60 members of the 97-person football team developed fever, malaise, loss of appetite, and abdominal discomfort. Within a few days, 30 players became jaundiced. Blood samples were drawn from all members of the team to test for antibodies to hepatitis A (the presumptive diagnosis) and to test for elevation of liver enzymes 1. What is the cumulative incidence of jaundice? . ??? 2. If you assume that all persons with symptoms had hepatitis A, even those that did not develop jaundice, what is the presumed cumulative incidence of hepatitis A? ??? 3. Laboratory testing revealed that 91 had elevated liver enzymes of which 90 had IgM antibody indicative of acute hepatitis A infection. Two players with normal liver enzymes had IgG antibody, indicating that they had previously been exposed to hepatitis A and are now immune. What is the cumulative incidence of hepatitis A? ??? Choices 30/60 30/97 60/97 90/91 90/95 90/97 91/95 91/97

A Pocket Guide to Epidemiology 53 Label each of the following statements as True or False: 4. Cumulative incidence is always a proportion, even for a cohort with staggered entry ("shifted cohort"). ??? 5. Cumulative incidence is a useful measure for diseases with short incubation periods in well-defined populations. ??? 6. Cumulative incidence is a less-than-ideal measure for diseases with long incubations periods in dynamic populations. ??? 7. If a fixed population has substantial loss-to-follow-up, cumulative incidence will overestimate the true risk of disease. ???

Rate
Rate is a measure of disease frequency that describes how rapidly health events such as new diagnoses of cases or deaths are occurring in a population of interest. Synonyms: hazard, incidence density.

Concept of a Rate
The concept of a rate is not as easily understood as risk, and is often confused with risk. Loosely speaking, a rate is a measure of how quickly something of interest happens. When we want to know how fast we are traveling in our car, how quickly the stock market prices are increasing, or how steadily the crime rate is decreasing, we are seeking a rate. Suppose we are taking a trip in a car. We are driving along an expressway and we look at our speedometer and see we are going 65 miles per hour. Does this mean that we will cover exactly 65 miles in the next hour? Of course not. The speedometer reading tells us how fast we are traveling at the moment of time we looked at the reading. If we were able to drive exactly this way for the next hour without stopping for gas or a rest or slowing down for heavy traffic, we would cover 65 miles in the next hour. The reading of 65 miles per hour on our speedometer is the velocity at which we are traveling, and velocity is an example of a rate. Actually, velocity is an example of an instantaneous rate, since it describes how fast we are traveling at a particular instant of time. There is another kind of rate, called an average rate, which we can also illustrate by continuing our car trip. If we actually traveled along the highway for the next hour and covered 55 miles during that time, the average rate, often called the speed that we traveled over the one-hour period, would be 55. In epidemiology, we use a rate to measure how rapidly new cases of a disease are developing, or alternatively, how rapidly persons with a disease of interest are dying. As with velocity or speed, we might want to know either the instantaneous rate or the average rate. With epidemiologic data, it is typically easier to determine an average rate than an instantaneous rate. We could hardly expect to have a speedometer-like device that measures how fast a disease is occurring at a particular moment of time in a cohort of subjects. Consequently, in epidemiologic studies, we typically measure the average rate at which a disease is occurring over a period of time. Because a rate is a measure of how quickly something is occurring, it is always

54 Chapter 4. How Often does it Happen? Disease Frequency measured in units of time, say, days, weeks, months, or years. This clarifies its interpretation. If we describe a rate of 50 new cases per 10,000 person-years, we mean that an average of 50 cases occurs for every 10,000 years of disease free follow-up time observed on a cohort of subjects. The 10,000 figure is obtained by adding together the follow-up times for all subjects in the cohort. If the unit of time was months instead of years, the interpretation of the rate can be quite different. A rate of 50 new cases per 10,000 person months indicates a much quicker rate than 50 new cases per 10,000 person years. Study Questions (Q4.6) 1. Which of the following rates is not equivalent to a rate of 50 new cases per 10,000 person years? A. 100 new cases per 20,000 person years B. 50 new cases per 120,000 person months C. 50 new cases per 52,000 person weeks 2. Determine whether or not each of the following statements describes a rate: A. 5 new cases per 100 person days Yes No Yes No B. 40 miles per hour C. 10 new cases out of 100 disease-free persons Yes No D. 60 new murders per year Yes No E. 60 deaths out of 200 clung cancer patients Yes No Summary Generally, a rate is a measure of how quickly something of interest is happening In epidemiology, a rate is a measure of how rapidly new cases of a disease or other outcome develop. An instantaneous rate, like velocity, describes how rapidly disease or death is occurring at a moment in time An average rate, like speed, describes how rapidly disease or death is occurring as an average over a period of time. In epidemiology, we typically use average rates rather than instantaneous rates. Rates must be measured in units of time.

A Pocket Guide to Epidemiology 55
Incidence Density- The Concept The term incidence density (ID) has been proposed by Miettinen to provide an intuitive interpretation of the concept of an average incidence rate. The diagram below illustrates incidence density as the concentration (i.e., density) of new case occurrences in an accumulation (or sea) of person-time. Person-time (PT) is represented by the area under the curve N(t) that describes number of disease-free persons at time t during a period of follow-up from time T0 to T1. Each new case is denoted by a small circle located within the sea of person-time at the time of disease occurrence. The concentration of circles within the sea represents the density of cases. The higher the concentration, the higher is the average rate during the period of follow-up.

Calculation of a Rate
To calculate a rate, we must follow a cohort of subjects, count the number of new (or incident) cases, I, of a disease in that cohort, and compute the total time, called person-time or PT, that disease-free individuals in the cohort are observed over the study period. The estimated incidence rate ( ˆR ) is obtained by dividing I by PT: I
ˆR I I PT

This formula gives an average rate, rather than the more difficult to estimate instantaneous rate. The formula is general enough to be used for any outcome of interest, including death. If the outcome is death instead of disease incidence, the formula gives the mortality incidence rate rather than the disease incidence rate. Consider again the following hypothetical cohort of 12 initially disease-free subjects followed over a 5-year period from 1990 to 1995. From these data, the number of new cases is 5. The total person-time, in this case person-years, is obtained by adding the individual observed disease-free follow-up times this gives a total of 25 person years. The incidence rate is therefore 5 divided by 25 or 0.20, which can be translated as 20 new cases per 100 person years of follow-up.

56 Chapter 4. How Often does it Happen? Disease Frequency Study Questions (Q4.7) 1. In this example, is the value of 0.20 a proportion? 2. In this example, does the value of 0.20 represent the risk of developing disease? 3. Which of the following rates is not equivalent to a rate of 20 new cases per 100 person years? A. 5 new cases per 25 years B. 40 new cases per 200 person years C. 480 new cases per 2400 person months D. 20 new cases per 1200 person months Summary A rate is calculated using the formula I/PT, where I denotes the number of incident cases and PT denotes the accumulated person-time of observed follow-up over the study period. This formula gives an average rate, rather than the more difficult to estimate instantaneous rate. A rate is always greater than zero and has no upper bound. The rate is always stated in units of person-time. A rate of .20 cases per person-year is equivalent to 20 cases per 100 personyears as well as 20 cases per 1200 person-months
The Big-Mac Assumption about Person-Time We have seen that the general formula for calculating an average rate (R) is:

R

I PT

where I is the number of new cases and PT is the accumulated person-time over a specified period of follow-up. When individually observed follow-up times are available, PT is determined by summing these individual times together for all N subjects in the disease-free cohort. For example, if 100 persons are each followed for 10 years, then PT=1000 personyears. Also, if 1000 persons are each followed for 1 year, we get PT=1000. A key assumption about PT is that both of these situations provide equivalent persontime information. In other words, the rate corresponding to a specified value of PT should not be affected by how the total person-time is obtained. We call this assumption the BigMac assumption because it is similar to assuming that eating 50 fast-food hamburgers costing $2.00 each is equivalent to eating: $100 gourmet meal at the best-rated restaurant in town. The Big-Mac assumption for PT will not hold, however, if the average time between first exposure and detection of the disease (i.e., the latency) is longer than the average individually observed follow-up time. If so, we would expect the rate to be lower in a large cohort that accumulates the same PT as a smaller cohort with larger individual follow-up times. For example, if the latency were 2 years, we would expect an extremely low rate for 1000 persons followed for one-year each but a much larger rate for 100 persons followed for two-years each. Individuals in the larger cohort would not be followed long enough to result in many new cases.

A Pocket Guide to Epidemiology 57

Determining Person Time Information
There are a number of ways to determine the person-time denominator in the formula for a rate. As illustrated in the previous section, when individual followup times are available on each person in the cohort, the person-time is calculated by summing ( ) individual follow-up times over the entire disease-free cohort:

ˆR I

I PT

When individual follow-up times are not available, one method for computing person-time information uses the formula:

PT = N* x t where N* is the average size of the disease-free cohort over the time period of study and t is the time length of the study period. This formula is particularly useful if the study cohort is a large population, such as a city, where individual person time information would be very difficult to obtain. For such a large cohort, it would also be difficult to exclude existing cases of the disease at the start of the study period as well as to determine the number of disease-free persons that are not followed for the entire period of study. Nevertheless, it may be that relatively few persons in the population develop the disease. And, we may be able to assume that the population is a stable dynamic cohort, that is, the population undergoes no major demographic shifts during the time period of interest. If so, the average size of the disease free cohort can be estimated by the size of the entire population based on census data available close to the time period of the study, which is what we have denoted N* in our persontime formula. As an example, suppose a stable population of 100,000 men is followed for a period of 5 years, during which time 500 new cases of bladder cancer are detected. The accumulated personyears for this cohort can then be estimated as 100,000 times 5, or 500,000 person-years. Consequently, the average incidence rate for the 5year period is given by 500 divided by 500,000, or 0.001 per year, or equivalently 1 new case per 1000 person years. Summary There are alternative ways to determine person-time information required in the denominator of a rate when individual follow-up times are not available. One method uses the formula PT = N* x t, where N* denotes the average size of a stable dynamic cohort based on census data available close to the chronological time of the study, and t is the time period of the study. This formula is useful if the study cohort is a large population for which individual person time information would be difficult to obtain.

58 Chapter 4. How Often does it Happen? Disease Frequency

Incidence Rate of Parkinson’s Disease
Parkinson 's disease is a seriously disabling disease characterized by a resting tremor, rigidity, slow movements, and disturbed reflexes. A cohort of more than 6,500 Dutch elderly people who did not have Parkinson 's disease at the start of the study was followed for six years to determine the incidence rate at which new cases of Parkinson 's disease develop. During the follow-up period, 66 participants were diagnosed with Parkinson 's disease. Because Parkinson 's disease has a subtle onset, it was difficult to determine exactly when the disease process had begun. Therefore, the investigators calculated the time of onset as the midpoint between the time of diagnosis and the time at which a participant was last known to be free of Parkinson 's. They could then calculate the total number of disease-free person-years in this study by adding up the number of person-years that each of the 6,500 participants had contributed to the study until he or she either: 1. Developed Parkinson 's disease 2. Died 3. Reached the end of the study period alive without having developed Parkinson 's disease. This resulted in a total of 38,458 disease-free person-years. In this study, the average incidence rate of Parkinson 's disease for the 6-year study period is: 66 / 38,458 = 0.0017 cases per person-year This means that, 1.7 new cases of Parkinson 's disease develop per 1,000 person-years. Study Questions (Q4.8) 1. Using the formula PT = N* x ( t), how many person-years would have been computed for this study population had no detailed information on each individual’s contribution to the total amount of person-years been available? 2. Using the number of person-years from the previous question, what is the incidence rate? Summary A cohort of more than 6,500 Dutch elderly people who did not have Parkinson’s disease at the start of the study was followed for six years to determine the rate at which new cases develop. The results indicate that 1.7 new cases of Parkinson’s disease develop for every 1,000 person-years of follow-up. The person-years calculation used the formula PT = N* x t since there was no detailed information on each individual’s person-years. Quiz (Q4.9) Label each of the following statements as True or False. 1. Rate is not a proportion ??? 2. Rate has units of 1/person-time, and varies from zero to one. ??? 3. A rate can only be calculated if every person in a cohort is followed individually to count and add up the person-time. ??? 4. Rate can be calculated for a dynamic cohort, but not for a fixed, stable cohort. ???

A Pocket Guide to Epidemiology 59

Risk versus Rate
Incidence can be measured as either risk or rate. Which of these types to use is an important choice when planning an epidemiologic study. We have seen two distinct measures for quantifying disease frequency, risk and rate. Risk is a probability, lying between 0 and 1 that gives the likelihood of a change in health status for an individual over a specified period of follow-up. Rate describes how rapidly new events are occurring in a population. An instantaneous rate, which is rarely calculated, applies to a fixed point in time whereas an average rate applies to a period of time. A rate is not a probability, is ) and is defined in always non-negative, has no upper bound (i.e. 0 < Rate < units of time, such as years, months, or days. When planning an epidemiologic study, which measure do we want to use, risk or rate? The choice depends on the objective of the study, the type of disease condition being considered, the nature of the population of interest, and the information available. If the study objective is to predict a change in health status for an individual, then risk is required. In particular, risk is relevant for assessing the prognosis of a patient, for selecting an appropriate treatment strategy, and for making personal decisions about health-related behaviors such as smoking, exercise, and diet. By contrast, a rate has no useful interpretation at the individual level. If the study objective is to test a specific hypothesis about disease etiology, the choice can be either risk or rate depending on the nature of the disease and the way we observe new cases. If the disease is a chronic disease that requires a long period of follow-up to obtain sufficient case numbers, there will typically be considerable loss to follow-up or withdrawals from the study. Consequently, individual observed follow-up times tend to vary considerably. A rate, rather than a risk, can address this problem. However, if an acute disease is considered, such as an outbreak due to an infectious agent, there is likely to be minimal loss to follow-up, so that risk can be estimated directly. With an acute illness, we are not so much interested in how rapidly the disease is occurring, since the study period is relatively short. Rather, we are interested in identifying the source factor chiefly responsible for increasing individual risk. If the population studied is a large dynamic population, individual follow-up times, whether obtainable or not, will vary considerably for different subjects, so rate must be preferred to risk. However, if individual follow-up times are not available, even a rate cannot be estimated unless it is assumed that the population size is stable, the disease is rare, and a recent census estimate of the population is available. Risk is often preferred to rate because it is easier to interpret. Nevertheless, rate must often be the measure of choice because of the problems associated with estimating risk. Summary Risk is the probability that an individual will develop a given disease over a specified follow-up period.

60 Chapter 4. How Often does it Happen? Disease Frequency Rate describes how rapidly new events are occurring in a population. Risk must be between 0 and 1 whereas rate is always non-negative with no upper bound, and is defined in units of time. Risk is often preferred to rate because it is easier to interpret. Rate must often be the measure of choice because of problems with estimating risk. The choice of risk versus rate depends on the study objective, the type of disease, the type of population, and the information available. Quiz (Q4.10) Do the following statements define a rate, risk, or both? 1. More useful for individual decision-making. 2. Numerator is number of new cases during a period of follow-up. 3. Lowest possible value is zero. 4. No upper bound. 5. Can be expressed as a percentage. 6. Better for studies with variable periods of follow-up. 7. Traditionally calculated in the acute outbreak (short follow-up) setting. 8. Measures how quickly illness or death occurs in a population. 9. Cumulative incidence. 10. Measure of disease occurrence in a population. ??? ??? ??? ??? ??? ??? ??? ??? ??? ???

Prevalence
Prevalence measures existing cases of a health condition and is the primary design feature of a cross-sectional study. There are two types of prevalence, point prevalence, which is most commonly used, and period prevalence. In epidemiology, prevalence typically concerns the identification of existing cases of a disease in a population and is the primary design feature of crosssectional studies. Prevalence can also more broadly concern identifying persons with any characteristic of interest, not necessarily a disease. For example, we may wish to consider the prevalence of smoking, immunity status, or high cholesterol in a population. The most common measure of prevalence is point prevalence, which is defined as the probability that an individual in a population is a case at time t, i.e.,
ˆ P C N (# of observed cases at time t) (Populatio n size at time t)

Point prevalence is estimated as the proportion of persons in a study population that have a disease at a particular point in time (C). For example, if there are 150 individuals in a population and, on a certain day, 15 are ill with the flu, the estimated prevalence for this population is 10%, i.e.,
ˆ P 15 150 10 %

Study Questions (Q4.11) 1. Is point prevalence a proportion? 2. A study with a large denominator, or one involving rare events, may result in very low prevalence. For example, suppose that 13 people from a population of size 406,245 had a particular disease at time t. What is the point prevalence

A Pocket Guide to Epidemiology 61 of this disease at time t? A. 0.0032 B. 32% C. 0.000032 D. 0.0000032 3. Which of the following expressions is equivalent to the point prevalence estimate of 0.000032? A. 3.2 per 1,000 B. 3.2 per 100,000 C. 32 per 100,000 When measuring point prevalence, it is essential to indicate when the cases were enumerated by specifying a point calendar time or a fixed point in a time sequence, such as the third post-operative day. Prevalence measures are very useful for assessing the health status of a population and for planning health services. This is because the number of existing cases at any time is a determinant of the demand for healthcare. However, prevalence measures are not as well suited as incidence measures, such as risk or rate, for identifying risk factors. This is because prevalence concerns only survivors, so that cases that died prior to the time that prevalence is measured are ignored. Summary Prevalence concerns existing cases of a disease at a point or period of time. Prevalence measures are primarily estimated from cross-sectional surveys. Point prevalence is the probability that an individual in a population is a case at time t. Point prevalence is estimated using the formula P = C/N, where C is the number of existing cases at time t, and N is the size of the population at time t. Prevalence measures are useful for assessing the health status of a population and for planning health services. Prevalence measures concern survivors, so they are not well suited for identifying risk factors.
Period Prevalence An alternative measure to point prevalence is period prevalence (PP), which requires the assumption of a stable dynamic population for estimation. PP is estimated as the ratio of the number of persons C* who were observed to have the health condition (e.g., disease) anytime during a specified follow-up period, say from times T0 to T1, to the size N of the population for this same period, i.e., the formula for period prevalence is:

PP

C * N

C N

I

where C denotes the number of prevalent cases at time T0 and I denotes the number of incident cases that develop during the period. For example, if we followed a population of 150 persons for one year, and 25 had a disease of interest at the start of follow-up and another 15 new cases developed during the year, the period prevalence for the year would be: PP = (25 + 15)/ 150 = .27, or 27%, whereas the estimated point prevalence at the start of the period is: P = 25/150 = .17, or 17% and the estimated cumulative incidence for the one year period is: CI = 15/125 = .12, or 12%

62 Chapter 4. How Often does it Happen? Disease Frequency Quiz (Q4.12) Label each of the following statements as True or False 1. Prevalence is a more useful measure for health planning than for etiologic ??? research. 2. Like cumulative incidence, prevalence is a proportion that may range from ??? zero to one. 3. Prevalence measures are most commonly derived from follow-up studies. ??? 4. Whereas incidence usually refers to occurrence of illness, injury, or death, prevalence may refer to illness, disability, behaviors, exposures, and genetic risk factors. ??? 5. The formula for point prevalence is: a. # new cases / # persons in population b. # new cases / # persons who did not have the disease at the starting point of observation c. # new cases / # person-time of follow-up d. # current cases / # persons in population e. # current cases / # persons who did not have the disease at the starting point of observation f. # current cases / # person-time of follow-up

Mortality
As with incidence measures of disease frequency, incidence measures of mortality frequency can take the form of risk or rate depending on the study design and the study goals. Mortality risk can be measured in a number of ways, including disease-specific mortality risk, all-causes mortality risk, and case-fatality risk. For each measure, the formula for simple cumulative incidence can be used. Here, I denotes the number of deaths observed over a specific study period in an initial cohort of size N.
Study Questions (Q4.13) 1. For a disease-specific mortality risk, what does the I in the formula CI=I/N represent. ??? A. The number of deaths from all causes B. The number of deaths due to the specific disease of interest C. The number of persons with a specific disease D. The size of the initial cohort regardless of disease status

Answer: For estimating disease-specific mortality risk, I is the number of deaths due to the specific disease of interest, and N is the size of the initial cohort regardless of disease status.

A Pocket Guide to Epidemiology 63 Study Questions (Q4.13) continued 2. For all-causes mortality risk, what does the I in the formula CI=I/N represent. ??? A. The number of deaths from all causes B. The number of deaths due to the specific disease of interest C. The number of persons with a specific disease D. The size of the initial cohort regardless of disease status

Answer: I is the number of deaths from all causes, and N is the size of the initial cohort, regardless of disease status.
Case-fatality risk is the proportion of people with a disease who die from that disease during the study period. Study Questions (Q4.13) continued 3. For case-fatality risk, what does the I in the formula CI=I/N represent. ??? A. The number of deaths from all causes B. The number of deaths due to the specific disease of interest C. The number of persons with a specific disease D. The size of the initial cohort regardless of disease status

Answer: I is the number of persons who die from the given disease, and N is the number of persons with this disease in the initial cohort. In a similar way, mortality rate can be measured using the general formula for average rate: IR = I/ PT. Here, I denotes the number of deaths observed over a specified study period in an initial cohort that accumulates person-time PT. For estimating disease-specific mortality rate, PT is the person-time for the initial cohort, regardless of disease condition. For estimating all-cause mortality rate, PT again is the person-time for initial cohort, regardless of disease condition. For estimating case-fatality rate, PT is the person time for an initial cohort of persons with the specific disease of interest that is followed to observe mortality status. As an example of the calculation of mortality risk estimates, suppose you observe an initial cohort of 1000 persons aged 65 or older for three years. One hundred out of the 1000 had lung cancer at the start of follow-up, and 40 out of these l00 died from their lung cancer. In addition, 15 persons developed lung cancer during the follow-up period and 10 died. Of the remaining 885 persons without lung cancer, 150 also died. The lung-cancer specific mortality risk for this cohort is 50/1000 or 5%. The all-cause mortality risk is 200/1000 or 20%, and The case-fatality risk for the 100 lung cancer patients in the initial cohort is 40/100 or 40%.

64 Chapter 4. How Often does it Happen? Disease Frequency Study Questions (Q4.14) For the lung cancer example just presented, answer the following questions: 1. From the data, what is the estimated risk for the incidence of lung cancer over the three-year period? 2. Why is the estimated incidence of lung cancer (LC) different from the estimated LC mortality of 5%? 3. Under what circumstances would you expect the LC incidence and LC mortality risk to be approximately equal? Summary Incidence measures of mortality frequency can take the form of risk or rate depending on the study design and the study goals. Mortality risk or rate can be measured in a number of ways, including diseasespecific mortality risk or rate, all-causes mortality risk or rate, and case-fatality risk or rate. For measuring mortality risk, the formula used for simple cumulative incidence, namely, CI = I / N, can be used. Similarly, mortality rate can be measured using the general formula for average rate, namely IR = I / PT. Quiz (Q4.15) During the past two years, a total of exactly 2,000 residents died in a retirement community with a stable, dynamic population of 10,000 persons. 1. Given these data, the best choice for measure of mortality is the mortality rate. ??? 2. Since mortality is often expressed per 1,000, one could express this mortality measure as 200 per 1,000 per year. ??? 3. The disease-specific mortality risk is the number of deaths attributable to a ??? particular disease, divided by the number of persons with that disease. 4. The denominator for the all-cause mortality risk and the cause-specific mortality risk is the same. ??? 5. The denominator for case-fatality risk is the numerator of the prevalence of the disease. ???

Age-adjusted rate
Most epidemiologic studies involve a comparison of measures of disease frequency among two or more groups. For example, to study the effect of climate conditions on mortality, we might compare mortality risks or rates in two or more locations with different climates. Let 's focus on two U.S. states, Arizona and Alaska. This would allow a comparison of mortality in a cold, damp climate with mortality in a hot dry climate. The crude mortality rates for these two states for the year 1996 were: Alaska 426.57 deaths per 100,000 population Arizona 824.21 deaths per 100,000 population You might be surprised, particularly considering the climates of the two states, that Arizona 's death rate is almost twice as high as Alaska 's. Does that mean that it 's far more hazardous to live in Arizona than Alaska?

A Pocket Guide to Epidemiology 65 Study Question (Q4.16) 1. What do you think? Is if far more hazardous to live in Arizona than Alaska?

A little knowledge of the demographic make-up of these two states might cause you to question such an interpretation. Look at the age distribution of the two states:

(Note: Alaska is the left bar for each of the clustered bars, Arizona the right bar.)

Study Questions (Q4.16) continued 2. Which population is older? 3. Why should we expect relatively more deaths in Arizona than in Alaska?

The variable age in this situation is called a confounder because it distorts the comparison of interest. We should correct for such a potentially misleading effect. One popular method for making such a correction is rate adjustment. If the confounding factor is age, this method is generally called age-adjustment, and the corrected rates are called age-adjusted rates. The goal of age adjustment is to modify the crude rates so that any difference in mortality rates of Alaska and Arizona cannot be explained by the age differences in the two states. The most popular method of rate adjustment is the direct method. This method forces the comparison of the two populations to be made on a common age distribution. The confounding factor age is removed by re-computing the rates substituting a common age distribution for the separate age distributions. The two populations are then compared as if they had the same age structure. The common age distribution is determined by identifying a standard population. A logical choice here would be the 1996 total United States population. Other choices for the standard are also possible and usually won 't make a meaningful difference in the comparison of adjusted rates.

(Note: The actual calculation of the age-adjusted rates is not shown here. For details on the calculation of the age-adjusted rates for this example, click on the asterisk on Lesson page 4-4 of the CD-ROM or see the example at the end of this chapter.)

66 Chapter 4. How Often does it Happen? Disease Frequency

The age-adjusted death rates obtained from using the direct adjustment method with the 1996 US population as the standard are shown here together with the crude rates: When we remove age as a factor, the age-adjusted death rate in Arizona is actually lower than in Alaska.
Study Questions (Q4.17) 1. How do we interpret these new age-adjusted results? 2. Based on these results, how do you think the age distribution of Alaska compares to that of the 1996 US population? 3. How do you think the age distribution of Arizona compares to that of the 1996 US population? Summary Comparing crude rates for two or more groups may be misleading because such rates do not account for the effects of confounding factors. If the confounding factor is age, this method is called age-adjustment, and the corrected rates are called age-adjusted rates. The goal of age adjustment is to modify the crude rates so that any difference in rates cannot be explained by age distribution of the comparison groups. The direct method of age-adjustment re-computes the rates by substituting a common age distribution for the separate age distributions of the groups being compared. The common age distribution is determined by identifying a standard population.
Terminology about Adjustment The rates described for Alaska and Arizona are actually risks. We have purposely used the term rates in this example to conform to the terminology typically used in published reports/papers that carry out age adjustment. In any case, the procedure used for (age) adjustment can be applied to any measure of disease frequency: risk, rate and/or prevalence. Moreover, potential confounding factors of interest other than age, e.g., race and sex, can also be adjusted, both individually and simultaneously. We generally use the term rate adjustment to describe adjustment involving any type or number of confounding factors and any type of measure of disease frequency, whether a risk, rate, or prevalence. Age-Adjustment – The Steps The method of direct age-adjustment involves the following steps: 1. Select a standard population whose age structure is known. By convention, the standard distribution used for age-adjustment of mortality rates in the United States is the US age distribution in the year closest to the year of the rates being compared. 2. Multiply the age-specific mortality rates for each group being compared by the corresponding age-specific numbers of persons in the standard population. The result is the expected number of deaths in each group. 3. Sum the expected numbers of deaths within each age group to yield a total number of expected deaths for each group being compared. 4. Divide the total number of expected deaths in each group by the total size of the standard population to yield summary age-adjusted mortality rates. An asterisk on lesson page 4-4 of the ActivEpi CD ROM provides a worked example of direct adjustment by comparing mortality rates for Alaska and Arizona in 1996.

A Pocket Guide to Epidemiology 67 Quiz (Q4.18) Label each of the following statements as True or False 1. Age-adjustment is a method to eliminate disparities in age between two ??? populations. 2. Age-adjustment always brings two disparate rates closer together ??? 3. When age-adjusting, one should use the U.S. population as the standard population. ??? 4. Age-adjustment can be used for one rate, two rates, or many rates. ??? 5. If the age distributions of two populations are very similar, their age-adjusted rates will also be similar. ??? 6. If the age distributions of two populations are very similar, the comparison of the age-adjusted rates will not be very different from the comparison of the ??? crude rates.

In the early 1990s, 7,983 elderly Dutch men and women were included in a prospective cohort study. The investigators computed how many person-years each participant had contributed to the study until January 2000. The total was 52,137 person-years. During follow-up, 2,294 of the participants died, and of these, 477 were due to coronary heart disease. 7. What is the all-cause mortality rate in this population? ??? per 1000 personyears. 8. What is the coronary heart disease-specific mortality rate? ??? per 1000 person-years Choices 2294 44 477 9.1 The crude all-cause mortality rate for men was 47.4 per 1000 person-years (PY) and for women was 41.9 per 1000 person-years. After making the age distribution in the women comparable to the age distribution in men (by standardizing the rates using the age distribution of the men), the mortality rate for women was only 27.8 per 1000 PY. 9. Based on these figures, the women must be considerably ??? than

the men in this population.
Choices C C* CI D I IR N P PP PT R T or t older younger

Nomenclature
Number of prevalent cases at time T C + I (number of prevalence cases at time T plus incident cases during study period) Cumulative incidence (“risk”): CI=I/N Duration of disease Incidence Incidence rate (“rate”): IR=I/PT Size of population under study Prevalence: P=C/N Period prevalence: PP=C*/N Person-time Average rate Time

68 Chapter 4. How Often does it Happen? Disease Frequency

References
General References
Greenberg RS, Daniels SR, Flanders WD, Eley JW, Boring JR. Medical Epidemiology (3rd Ed). Lange Medical Books, New York, 2001. Kleinbaum DG, Kupper LL, Morgenstern H. Epidemiologic Research: Principles and Quantitative Methods. John Wiley and Sons Publishers, New York, 1982. Ulm K. A simple method to calculate the confidence interval of a standardized mortality ratio (SMR). Am J Epidemiol 1990;131(2):373-5.

References on Rates
Miettinen O. Estimability and estimation in case-referent studies. Am J Epidemiol 1976;103(2):226-35 Giesbergen PCLM, de Rijk MC, van Swieten JC, et al. Incidence of parkinsonism and parkinson 's disease in a general population: the Rotterdam Study. Am J Epidemiol (in press).

References on Age-Adjustment
Dawson-Saunders B, Trapp RG. Basic and Clinical Biostatistics, 2nd ed., Appleton and Lange, Stamford, CN, 1994. Woodward M. Epidemiology: Study Design and Analysis, Chapter 4, pp. 157-167, Chapman and Hall, Boca Raton, FL, 1999.

Reference for Prevalence and Incidence of HIV:
Horsburgh CR, Jarvis JQ, McArthur T, Ignacio T, Stock P. Serconversion to human immunodeficiency virus in prison inmates. Am J Public Hlth 1990;80(2):209-210.

Answers to Study Questions and Quizzes
Q4.1 1. Incidence – Here we are interested in the number of new cases after eating the potato salad. 2. Prevalence – Here we are interested in the number of existing cases. 3. Incidence – Here we are interested in the number of new cases that occur during the follow-up. 4. Incidence – Here we are interested in the number of new deaths attributed to the hurricane. 5. Prevalence – Here we are interested in the existing number of children who have immunity to measles. 6. Incidence – Since rabies has a short duration, we would expect the prevalence on a particular day to be low relative to the incidence. 7. Prevalence – The incidence of multiple sclerosis would be low, but since it has a long duration, we would expect the prevalence to be higher. 8. Incidence – The incidence of influenza would be high, but since it is of short duration the prevalence would be low. 9. Incidence – Since the duration of poison ivy is relatively short the prevalence would be low, and since it is a common occurrence, the incidence would be high. 10. Prevalence – Since high blood pressure is common and of long duration, both incidence and prevalence are high, but the prevalence would be higher. Q4.2 1. The statement means that a 45-year-old male free of prostate cancer has a probability of .05 of developing prostate cancer over the next 10 years if he does not die from any other cause during the follow-up period. 2. Smaller, because the 5-year risk involves a shorter time period for the same person to develop prostate cancer. Q4.3 1. No, subjects should be counted as new cases if they were disease-free at the start of follow-up and became a case at any time during the follow-up period specified. 2. Yes, there is a problem, since a subject followed for 2 years does not have the same opportunity for developing the

A Pocket Guide to Epidemiology 69 disease as a subject followed for 4 years. 3. No, but we must assume that subjects who do not develop the disease have the same follow-up time. Otherwise, we can get a misleading estimate of CI because not all subjects will have the same opportunity to develop the disease over the follow-up period. Q4.4 1. Dynamic 2. Subject 2 3. Subject 7 4. Subject 9 5. Subject 3 6. Subject 5 7. 5/12=42% 8. 4/12=33% 9. D 10. No 11. No 12. Yes 13. Yes 14. Yes 15. No 16. Yes Q4.5 1. 30/97 2. 60/97 3. 90/95 4. True – The numerator of the CI formula is a subset of the denominator. 5. True – Because the incubation period is short, subjects are not likely to be lost to follow-up. 6. True – The long incubation period means subjects are likely to be lost to follow-up, and hence cases may not be detected. For a dynamic cohort, the denominator in the CI formula does not reflect the continually changing population size. 7. False – the estimated CI will underestimate the true risk of disease. Q4.6 1. C 2. a. Yes b. No c. Yes d. No Q4.7 1. No, the denominator of 25 does not describe 25 persons, but rather the accumulated follow-up time for 12 persons. 2. No, the risk in this example would be calculated as 5/12 or 0.42. However, using risk would be questionable here because different subjects have different follow-up times. 3. C Q4.8 1. N* is the average size of the diseasefree cohort and t is the time length of the study period. Therefore, a rough estimate of the total amount of personyears contributed by the study is 6,500 *6 = 39,000 person-years. 2. The incidence rate is 66/39,000 = 0.0017, or 1.7 per 1,000 person-years. Q4.9 1. True – For questions 1 & 2: a rate can range from 0 to infinity, whereas a risk (which is a proportion) ranges from 0 to 1 (or 0% to 100%). 2. False 3. False – There are alternative ways to calculate person-time information when individual follow-up time is unavailable. 4. False – A rate can be calculated for either a dynamic cohort or fixed cohort, depending on the person-time information available. Q4.10 1. Risk 2. Both 3. Both 4. Rate 5. Risk 6. Rate 7. Risk 8. Rate 9. Risk 10. Both Q4.11 1. Yes, its value can range from 0 to 1 and it is often expressed as a percentage 2. C. The prevalence of disease is 13/406,245 = 0.000032 3. B. 3.2 per 100,000 is an equivalent expression and is easier to interpret Q4.12 1. True – Prevalence considers existing cases rather than incident cases.

70 Chapter 4. How Often does it Happen? Disease Frequency
2. True – Since the numerator is contained in the denominator, prevalence is a proportion and must range from 0 to 1 (or 0% to 100%). 3. False – Cross-sectional studies are carried out at essentially a single (or short) point in time. 4. True – Prevalence may concern a health outcome or any other characteristic of a subject. 5. d Q4.13 1. B 2. A 3. B Q4.14 1. The estimate of LC incidence is calculated as CI = 15/900 = .017 or 1.7% 2. The 5% mortality estimate counts the 40 prevalent LC cases and does not count the 5 new LC cases that did not die. Furthermore, the denominators are different. 3. The LC incidence and mortality risks would be about equal if the disease was quickly fatal, so that there would be few if any prevalent cases in the initial cohort and all new cases would have died before the end of follow-up. Q4.15 1. True 2. True 3. False – The denominator of a diseasespecific mortality risk is the size of the initial cohort regardless of disease status. 4. True 5.True Q4.16 1. The two rates are crude rates because they represent the overall mortality experience in 1996 for the entire population of each state. Crude rates do not account for any differences in these populations on factors such as age, race, or sex that might have some influence on mortality. Without consideration of such factors, it would be premature to make such a conclusion. 2. Arizona. The dry, warm climate of Arizona attracts many older persons than does Alaska. 3. There are relatively older persons living in Arizona, and older persons are at high risk of dying. Q4.17 1. Controlling for any age differences in the two populations, the overall mortality rate is higher in Alaska with a cold, damp climate, then in Arizona where the climate is warm and dry. 2. The population of Alaska must be much younger than the US population since the age-adjusted rate was so much higher than the crude rate. 3. The rate for Arizona did not change much from crude to adjusted because Arizona’s age distribution was only slightly younger than that of the entire US in 1996. Q4.18 1. True – If age-adjustment is not used, then a difference in risk or rates between two populations may be primarily due to age differences in the two populations. 2. False – There is no guarantee that two adjusted measures will be either closer or further from each other than were corresponding crude measures. 3. False – The choice of standard population depends on the characteristics of the populations being considered. 4. True – There is no limitation on the number populations that could be ageadjusted. 5. False – For questions 5 & 6: If the crude rates are quite different whereas the age distributions are similar, then the adjusted rates are likely to be quite different. 6. True 7. 44 8. 9.1 9. older – Women must be older than men in this case. The mortality rate drops substantially in women when we standardize the rate using the age distribution of men. In other words, if we take age out of the picture, the rates for women drop. If the women were younger we would expect to see the adjusted rate increase once we remove age as a factor.

CHAPTER

5

WHAT’S THE ANSWER? MEASURES OF EFFECT
In epidemiologic studies, we compare disease frequencies of two or more groups using a measure of effect. We will describe several types of measures of effect in this chapter. The choice of measure typically depends on the study design being used.
Ratio Versus Difference Measures of Effect Our focus in Chapter 5 is on ratio measures of effect, which are of the form Ml/M0, where Ml and M0 are two measures of disease frequency, e.g., risks, rates, or prevalences that are being compared. We consider difference measures of effect, which are of the form Ml-M0, in Chapter 6 on "Measures of Potential Impact". Difference measures are also called measures of attributable risk. Ratio measures are typically used in epidemiologic studies that address the etiology of a disease/health outcome, whereas difference measures are used to quantify the public health importance of factors that are determinants of a disease/health outcome.

Smoking and Lung Cancer
Cigarette smoking became increasingly popular in America after World War I when cigarettes were handed out to soldiers as a way to boost morale. But along with the rise in smoking, came a disturbing rise in the lung cancer rate and some early warnings from a handful of doctors about possible dangers of smoking. Early studies in the 1930s and 1940s of the possible relationship between smoking and lung cancer were case-control studies. It became quite apparent that lung cancer patients smoked much more than controls. In one study in particular, lung cancer patients were 17 times more likely than controls to be two-pack-a-day smokers. In the early 1950s, doctors Horn and Hammond of the American Cancer Society conducted one of the first cohort studies on the harmful effects of smoking. About 200,000 people were given a smoking questionnaire and then followed for four years. Death rates and cause of death for smokers and for nonsmokers were compared. The preliminary study published in 1958 caused quite a sensation. It was the largest study on smoking that had been done, and it showed that smokers were ten times more likely than nonsmokers to get lung cancer. Both the cohort and case-control studies attempted to assess the proposed relationship between smoking and lung cancer by deriving a measure of effect that quantified the extent of this relationship. The measure described in the casecontrol study is called an odds ratio. The measure described in the cohort study is called a risk ratio. The activities that follow discuss these two fundamental measures of effect. Summary The odds ratio and the risk ratio are two fundamental measures of effect. These measures were used in epidemiologic studies of the relationship between smoking and lung cancer.

72 Chapter 5. What’s the Answer? Measures of Effect The odds ratio is typically the measure of effect used in case-control studies. The risk ratio is typically the measure of effect used in cohort studies.

The Risk Ratio
The table below summarizes the results of a five-year follow-up study to determine whether or not smokers who have had a heart attack will reduce their risk for dying by quitting smoking. A cohort of 156 heart attack patients was studied, all of whom were regular smokers up to the time of their heart attack. Seventy-five of these patients continued to smoke after their attack. The other 81 patients quit smoking during their recovery period. Of the 75 patients that continued smoking, 27 died, so the proportion of these patients that died is 0.36. Of the 81 patients who quit smoking, 14 died, so the corresponding proportion is 0.17. These proportions estimate the fiveyear risks of dying for these two groups of patients. We may wonder whether those heart attack patients who continue smoking are more likely to die within 5 years after their first heart attack than those who quit. A measure of effect gives a numerical answer to this question. Such a measure allows us to make a comparison of two or more groups, in this case, continuing smokers and smokers who quit. For follow-up studies such as described here, the typical measure of effect is a risk ratio. To calculate a risk ratio, we take the ratio of the two risks being compared, that is, we simply divide one risk by the other. Actually, we are getting an "estimate" of the risk ratio, which we indicate by

ˆ putting a "hat" symbol over the RR notation. RR is an estimate because we are using two estimates of risk based on samples from the two groups being compared. In our example, therefore, we divide 0.36 by 0.17 to get 2.1.
ˆ Estimated RR Estimated Risk for continuing smokers Estimated Risk for smokers who quit 0.36 0.17 2.1

The estimated risk ratio of 2.1 tells us that continuing smokers are about twice as likely to die as smokers who quit. In other words, for heart attack patients the five-year risk for continuing smokers is about twice the corresponding risk for smokers who quit. Study Questions (Q5.1) Using the five-year follow-up study comparing mortality between smokers and quitters example: 1. How would you interpret a Risk Ratio of 4.5? 2. What if the Risk Ratio was 1.1? 3. How about if the Risk Ratio was less than 1, say 0.5? 4. How would you interpret a value of 0.25? If our estimated risk ratio had been 1.1, we would have evidence that the risk for continuing smokers was essentially equal to the risk for smokers who quit. We

A Pocket Guide to Epidemiology 73 call a risk ratio of 1 the null value of the risk ratio. This is the value that we get for the risk ratio when there is no effect, that is, the effect is null. Summary The risk ratio (RR) is the ratio of the risk for one group, say group 1, to the risk for another group, say group 0. The value of RR can be greater than one, equal to one, or less than one. If the RR is greater than one, the risk for group 1 is larger than the risk for group 0. If the RR is below one, the risk for group 1 is less than the risk for group 0. And, if the RR is equal to 1, the risks for group 1 and 0 are equal, so that there is no effect of being in one group when compared to the other.
Risk Ratio Numerator and Denominator In general, the risk ratio that compares two groups is defined to be the risk for one group divided by the risk for the other group. It is important to clearly specify which group is in the numerator and which group is in the denominator. If, for example, the two groups are labeled group 1 and group 0, and the risk for group 1 is in the numerator, then we say that the risk ratio compares group 1 to group 0. On the other hand, if the risk for group 0 is in the numerator, then we say that the risk ratio compares group 0 to group 1.

Quiz (Q5.2) For heart attack patients, the risk ratio is defined to be the risk for continuing smokers divided by the risk for smokers who quit. For the following scenarios what would be the risk ratio? 1. Continuing smokers are twice as likely to die as smokers who quit. 2. Continuing smokers are just as likely to die as smokers who quit. 3. Smokers who quit are twice as likely to die as continuing smokers. Choices 0 0.1 0.2 0.5 1 2 ??? ??? ???

Let 's consider the data from a randomized clinical trial to assess whether or not taking aspirin reduces the risk for heart disease. The exposed group received aspirin every other day whereas the comparison group received a placebo. A table of the results is shown below. Aspirin n Yes Developed Heart Disease No Total 104 9,896 10,000
Column %

Placebo n 189 7,811 8,000
Column %

Total 293 17,707 18,000

(1.04) (98.96) (100.00)

(2.36) (97.64) (100.00)

4. The estimated risk for the aspirin group is ??? 5. The estimated risk for the placebo group is ??? 6. The estimated risk ratio that compares the aspirin group to the placebo group is given by ??? Choices 0.0104 0.0236 0.44 104/189 2.269 98.96/97.64

74 Chapter 5. What’s the Answer? Measures of Effect

The Odds Ratio
Epidemiologists in the Division of Bacterial Diseases at CDC, the Centers for Disease Control and Prevention in Atlanta, investigate the sources of outbreaks caused by eating contaminated foods. For example, a case-control study was carried out to determine the source of an outbreak of diarrheal disease at a Haitian Resort Club from November 30 to December 8, 1984. The investigators wondered whether eating raw hamburger was a primary source of the outbreak. Because this is a case-control study rather than a follow-up study, the study design starts with cases, here, persons at the resort who had diarrhea during the time period of interest. The controls were a random sample of 33 persons who stayed at the resort but did not get diarrhea during the same time period. There were a total of 37 cases during the study period. All 37 cases and the 33 controls were interviewed by a team of investigators as to what foods they ate during their stay at the resort. Of the 37 cases, 17 persons ate raw hamburger, so that the proportion of the cases that ate raw hamburger is 0.46. Of the 33 controls, 7 ate raw hamburger, so the corresponding proportion is 0.21. We may wonder, then, whether these data suggest that eating raw hamburger was the source of the outbreak. Because this is a case-control study rather than a follow-up study, these proportions do not estimate risks for cases and controls. Therefore, we cannot compute a risk ratio. So, then, what measure of effect should be used in casecontrol studies? The answer is the odds ratio (OR), which is described in the next section. Summary A case-control study was used to investigate a foodborne outbreak at a Caribbean resort. In a case-control study, we cannot estimate risks for cases and controls. Consequently, we cannot use the risk ratio (RR) as a measure of effect, but must use the odds ratio (OR) instead.
Why can 't we use a risk ratio in case-control studies? In a case-control study, we cannot estimate risk, but rather, we estimate exposure probabilities for cases and controls. The exposure probability for a case is the probability that a subject is exposed given that he/she is a case; this is not equivalent to the probability that a subject is a case given that he/she is exposed, which is the risk for exposed. In other words, using conditional probability notation: Pr(exposed | case) Pr(case | exposed), where "|" denotes "given". Similarly the exposure probability for a control is not equivalent to 1 minus the risk for exposed. That is, Pr(exposed | control) 1 - Pr(case | exposed). The ratio of two exposure probabilities is, unfortunately, not a risk ratio. Therefore, in case-control studies we must use a different measure of effect, namely the odds ratio.

A Pocket Guide to Epidemiology 75

The Odds Ratio (continued)
To understand odds ratios, we must start with the concept of an odds. The term odds is commonly used in sporting events. We may read that the odds are 3 to 1 against a particular horse winning a race, or that the odds are 20 to 1 against Spain winning the next World Cup, or that the odds are 1 to 2 that the New York Yankees will reach the World Series this year. When we say that the odds against a given horse are 3 to 1, what we mean is that the horse is 3 times more likely to lose than to win. The odds of an event are easily calculated from its probability of occurrence. The odds can be expressed as P, the probability that the event will occur, divided by 1 - P, the probability that the event will not occur.

In our horse race example, if P denotes the probability that the horse will lose, then 1 - P denotes the opposite probability that the horse will win. So, if the probability that the horse will lose is 0.75, then the probability that the horse will win is 0.25, and the odds are 3, or 3 to 1.

Odds

P 1- P

P(horse will lose) P(horse will win)

0.75 0.25

3 or

3 1

In the Haitian resort case-control study, recall that the event of interest occurs if a study subject ate raw hamburger, and, if so, we say this subject is exposed. The estimated probability of exposure for the cases was 0.46, so the estimated odds of being exposed for cases is 0.46 divided by 1 - 0.46:

ˆ Odds Cases

0.46 1 - 0.46

.85

Similarly, the estimated probability of exposure for controls was 0.21, so the estimated odds for controls is 0.21 divided by 1 - 0.21:

ˆ Odds Controls

0.21 .27 1 - 0.21
ˆ Odds Cases ˆ Odds

The estimated odds ratio for these data is the ratio of the odds for cases divided by the odds for controls, which equals 3.2.

ˆ Odds Ratio (OR)

Controls

.85 .27

3.2

How do we interpret this odds ratio estimate? One interpretation is that the exposure odds for cases is about 3.2 times the exposure odds for controls. Since those who ate raw hamburger are the exposed subjects, the odds that a case ate raw hamburger appear to be about 3.2 times the odds that a control subject ate raw hamburger. Study Questions (Q5.3) Using the Haiti case-control study example: 1. How would you interpret an odds ratio of 2.5? 2. What if the odds ratio was 1.1?

76 Chapter 5. What’s the Answer? Measures of Effect 3. How about if the odds ratio less than 1, say 0.5? 4. How would you interpret a value of 0.25? Odds ratios, like risk ratios, can be greater than one, equal to one, or less than one. An odds ratio greater than one says that the exposure odds for cases is larger than the exposure odds for controls. An odds ratio below one says that the exposure odds for cases is less than the exposure odds for controls. An odds ratio equal to 1 says that the exposure odds for cases and controls are equal. Summary The odds of an event can be calculated as P/(1-P) where P is the probability of the event. The odds ratio (OR) is the ratio of two odds. In case-control studies, the OR is given by the exposure odds for the cases divided by the exposure odds for controls. Odds ratios, like risk ratios, can be greater than 1, equal to 1, or less than 1, where 1 is the null value. Quiz (Q5.4) A causal relationship between cigarette smoking and lung cancer was first suspected in the 1920s on the basis of clinical observations. To test this apparent association, numerous studies were conducted between 1930 and 1960. A classic case-control study was done in 1947 to compare the smoking habits of lung cancer patients with the smoking habits of other patients. 1. In this case-control study, it is ??? to calculate the risk of lung cancer among smokers, and thus, the appropriate measure of association is the ???. Choices Not possible odds ratiopossible risk ratio Let 's consider the data below from this classic case-control study to assess the relationship between smoking and lung cancer. Cases were hospitalized patients newly diagnosed with lung cancer. Controls were patients with other disorders. This 2 x 2 table compares smoking habits for the male cases and controls. 2. 3. 4. 5. 6. The probability of being a smoker among cases is The probability of being a smoker among controls is The odds of smoking among cases is The odds of smoking among controls is The odds ratio is 10.50 1296/1357 1350/1357 1350/2646 192.86 9.08
Non-Smoker Total

??? ??? ??? ??? ???

Choices 0.11 1.04 21.25 7/68 Cases Controls Total

Cigarette Smoker

1350 1296 2646

7 61 68

1357 1357 2714

In a case-control study to find the source of an outbreak, the odds ratio for eating coleslaw is defined to be the odds for cases divided by the odds for controls. For

A Pocket Guide to Epidemiology 77 the following scenarios what would be the odds ratio? 7. Cases have an odds for eating coleslaw three times higher than controls 8. Cases have the same odds for eating coleslaw as controls 9. Controls have three times the odds for eating coleslaw as cases Choices 0 0.25 0.333 1 3 4 ??? ??? ???

Calculating the Odds Ratio
This layout for a two by two table provides a more convenient way to calculate the odds ratio. The formula is a times d over b times c. It is called the cross product ratio formula because it is the ratio of one product that crosses the table divided by the other product that crosses the table. To illustrate this formula consider the data from the Haitian resort outbreak. The cross product formula gives us the same result, 3.2, as we obtained originally from the ratio of exposure odds for cases and controls.

Study Question (Q5.5) 1. Should we calculate the OR for other foods eaten during the outbreak before we blame raw hamburger as the source? Although the odds ratio must be computed in case-control studies for which the risk ratio cannot be estimated, the odds ratio can also be computed in follow-up studies. (Note that the OR and RR can also be calculated in randomized clinical trials that have cumulative incidence measures.) For example, let us consider the "quit smoking" study for heart attack patients. The study design here is a follow-up study. We previously estimated that the risk for patients who continued to smoke was 2.1 times greater than the risk for those who quit. Using the cross product formula on these follow-up data

78 Chapter 5. What’s the Answer? Measures of Effect yields 2.7. The fact that these two numbers (the risk ratio and odds ratio) are not equal should not be surprising, since the risk ratio and odds ratio are two different measures. But the values in this example are not very different. In fact, these two estimates have similar interpretations since they both suggest that there is a moderate relationship between quit smoking status and survival status. Summary A convenient formula for the OR is the cross product ratio: (ad)/(bc) The OR can be estimated in both case-control and follow-up studies using the cross-product formula.
(See below for discussion of how the risk ratio can be approximated by the odds ratio.)

Quiz (Q5.6) To study the relationship between oral contraceptive use and ovarian cancer, CDC initiated the Cancer and Steroid Hormone Study in 1980 (see table below). It was a case-control study. 1. Using the cross product ratio formula, the OR comparing the exposure status of cases versus controls is (93) * (???) / (???) * (959) which equals ???. 2. This means that the ??? of ??? among the cases was ??? the ??? of exposure among the ???. Choices 0.23 0.77 1.3 683 86 cases controls disease exposed exposure greater than less than non-exposed odds risk Cases Controls Total Ever Used OCs 93 959 1052 Never Used OCs 86 683 769 Total 179 1642 1821

The Odds Ratio in Different Study Designs
The odds ratio can be computed for both case-control and follow-up (cohort) studies. Because a case-control study requires us to estimate exposure probabilities rather than risks, we often call the odds ratio computed in casecontrol studies the exposure odds ratio (EOR). In contrast, because a follow-up study allows us to estimate risks, we often call the odds ratio computed from follow-up studies the risk odds ratio (ROR). The odds ratio can also be computed for cross-sectional studies. Since a crosssectional study measures prevalence or existing conditions at a point in time, we usually call an odds ratio computed from a cross-sectional study a prevalence odds ratio (POR). As an example of the computation of a prevalence odds ratio for crosssectional data, consider these data that were collected from a cross-sectional

A Pocket Guide to Epidemiology 79 survey designed to assess the relationship between coronary heart disease and various risk factors, one of which was personality type. For these cross-sectional data, we can use the general cross product ratio formula to compute a prevalence odds ratio. The odds of having a type A personality among those with coronary heart disease is 5 times the odds of those without the disease. In general we can use the cross product ratio formula to compute an exposure odds ratio, a risk odds ratio, or a prevalence odds ratio depending on the study design used. Summary The OR computed from a case-control study is called the exposure odds ratio (EOR). The OR computed from a follow-up study is called the risk odds ratio (ROR) The OR computed from a cross-sectional study is called the prevalence odds ratio (POR) We can use the general cross-product ratio formula to calculate the EOR, ROR, or POR depending on the study design used.
Does ROR = EOR = POR? Not necessarily. Although the calculation formula (i.e., ad/bc) is the same regardless of the study design, different values of the estimated odds ratio from a 2 x 2 table might be obtained for different study designs. This is because of the possibility of selection bias (described in Chapter 8). For example, a case-control study that uses prevalent cases could yield a different odds ratio estimate than a follow-up study involving only incident cases.

Quiz (Q5.7) Data is shown below for a cross-sectional study to assess whether maternal cigarette smoking is a risk factor for low birth weight. 1. Calculate the odds ratio that measures whether smokers are more likely than non-smokers to deliver low birth weight babies. OR=??? 2. This odds ratio estimate suggests that smokers are ??? than non-smokers to have low birth weight babies. 3. This odds ratio is an example of a(n) ??? odds ratio. Choices 0.48 2.04 2.18 exposure prevalence risk Smokers 1,556 694 2,250 less likely more likely Total 16,530 15,226 31,756

Low Birth weight High Birth weight Total

Non-Smokers 14,974 14,532 29,506

80 Chapter 5. What’s the Answer? Measures of Effect

Comparing the Risk Ratio and the Odds Ratio in Follow-up Studies
We have described two widely used measures of effect, the risk ratio and the odds ratio. Risk ratios are often preferred because they are easier to interpret. But, as we have seen, in case-control studies, we cannot estimate risks and must work instead with an exposure odds ratio (EOR). In follow-up studies, however, we have the option of computing both a risk ratio and a risk odds ratio (ROR). Which should we prefer? It can be shown mathematically that if a risk ratio estimate is equal to or greater than one, then the corresponding risk odds ratio is at least as large as the risk ratio. For example, using the follow-up data for the quit smoking study of heart attack patients, we saw that the estimated risk ratio was 2.1, which is greater than one; the corresponding odds ratio was 2.7, which is larger than 2.1.

ˆ ˆ If RR 1, then ROR

ˆ RR

Similarly if the risk ratio is less than one, the corresponding odds ratio is as small or smaller than the risk ratio. For example, if we switch the columns of the quit smoking table, then the risk ratio is 0.48, which is less than one, and the corresponding odds ratio is 0.37, which is less than 0.48.

ˆ ˆ If RR 1, then ROR

ˆ RR

It can also be shown that if a disease is "rare", then the risk odds ratio will closely approximate the risk ratio. For follow-up studies, this rare disease assumption means that the risk that any study subject will develop the disease is small enough so that the corresponding odds ratio and risk ratio estimates give essentially the same interpretation of the effect of exposure on the disease. Typically a rare "disease", is considered to be a disease that occurs so infrequently in the population of interest that the risk for any study subject is approximately zero. For example, if one out of every 100,000 persons develops the disease, the risk for this population is zero to 4 decimal places. Now that 's really rare!

A Pocket Guide to Epidemiology 81 Study Questions (Q5.8) 1. Is a risk of .01 rare? 2. Suppose that for a given follow-up study, the true risk is not considered to be rare. Is it possible for the ROR and RR to be approximately the same? We can write a formula that expresses the risk odds ratio in terms of the risk ratio: ROR RR x f where (1 - R 0 ) f
(1 R 1)

and R0 is the risk for the unexposed, R1 is the risk for the exposed, and RR=R1/R0 This formula says that the risk odds ratio is equal to the risk ratio multiplied by the factor f, where f is defined as 1 minus the risk for the unexposed group (R0) divided by 1 minus the risk for the exposed group (R1). You can see from this equation that if both R1 and R0 are approximately 0, then f is approximately equal to one, and the risk odds ratio is approximately equal to the risk ratio. Study Questions (Q5.9) 1. In the quit smoking example, where R0 is 0.17 and R1 equals 0.36, what is f? 2. For this value of f, is the ROR close to the RR? 3. What happens to f if the risks are halved, i.e., R0 = 0.17/2 = 0.085 and R1 = 0.36/2 = 0.180? 4. Are the ROR and RR estimates close for this f? 5. What happens to f if we again halve the risks, so that R0=0.0425 and R1=0.09? 6. Is the approximation better? 7. Based on your answers to the above questions, how “rare” do the risks have to be for the odds and risk ratios to be approximately equal? Summary If an estimate of RR > 1, then the corresponding estimate of ROR is at least as large as the estimate of the RR. If an estimate of RR < 1, then the corresponding estimate of ROR is as small or smaller than the estimate of RR. In follow-up (cohort) studies, the “rare disease assumption” says that the risk for any study subject is approximately zero. Under the rare disease assumption, the risk odds ratio (ROR) computed in a follow-up study approximates the risk ratio (RR) computed from the same study.

Comparing the RR and the OR in the Rotterdam Study
Osteoporosis is a common disease in the elderly, and leads to an increased risk of bone fractures. To study this disease, a cohort consisting of nearly 1800 postmenopausal women living in Rotterdam, the Netherlands, was followed for four years. The Rotterdam Study investigators wanted to know which genetic factors determine the risk of fractures from osteoporosis. They focused on a gene coding for one of the collagens that are involved in bone formation. Each person 's genetic make-up consists of two alleles of this gene, and each allele can have one of two alternative forms, called allele A or allele B. The investigators showed that

82 Chapter 5. What’s the Answer? Measures of Effect women with two A alleles had a higher bone mass than women with at least one B allele. They therefore hypothesized that the risk of fractures would be higher in women with allele B. Of the 1194 women with two A alleles, 64, or 5.36%, had a fracture during follow-up. Of the 584 women with at least one B allele, 47, or 8.05%, had a fracture. Study Questions (Q5.10) 1. Calculate the risk ratio for the occurrence of fractures in women with at least one B allele compared to women with two A alleles. Because the risk ratio estimate is greater than 1, we expect the risk odds ratio to be at least as large as the risk ratio. Study Questions (Q5.10) continued 2. Calculate the risk odds ratio for the occurrence of fractures in women with at least one B allele compared to women with two A alleles. Note that the risk of fractures is relatively rare in this population; therefore the risk odds ratio is approximately equal to the risk ratio. Recall the formula ROR = RR * f. Here, f is defined as 1 minus the risk in women with two A alleles divided by 1 minus the risk in women with at least one B allele. Study Questions (Q5.10) continued 3. Using the formula ROR = RR x f, can you show that we computed the correct risk odds ratio? In this study, both the risk ratio and the risk odds ratio lead to the same conclusion: women with at least one B allele have a 50% higher chance of fractures than women with two A alleles. The Rotterdam Study investigators concluded that genetic make-up can predispose women to osteoporotic fractures. Quiz (Q5.11): RR versus OR in follow-up studies A questionnaire was administered to persons attending a social event in which 39 of the 87 participants became ill with a condition diagnosed as salmonellosis. The 2 x 2 table below summarizes the relationship between consumption of potato salad and illness. 1. 2. 3. 4. The risk ratio comparing the exposed to the non-exposed is The odds ratio is Does the odds ratio closely approximate the risk ratio? Do you consider this illness to be “rare”? 0.25 1.7 3.7 36.0 9.8 no yes Total 39 48 87 ??? ??? ??? ???

Choices Ill Well Total

Exposed 36 12 48

Non-Exposed 3 36 39

A Pocket Guide to Epidemiology 83 Let 's consider data from a classic study of pellagra. Pellagra is a disease caused by dietary deficiency of niacin and characterized by dermatitis, diarrhea, and dementia. Data comparing cases by gender are shown below. 5. 6. 7. 8. The risk ratio of pellagra for females versus males is (1 decimal place) The odds ratio is (to one decimal place) Does the odds ratio closely approximate the risk ratio? Do you consider this illness to be "rare"? 1.4 2.4 2.5 24.2 no Males 18 1401 1419 yes Total 64 2839 2903 ??? ??? ??? ???

Choices Ill Well Total

Females 46 1438 1484

Comparing the RR and OR in Case-Control Studies
We have already seen that, for follow-up studies, if the disease is "rare", then the risk odds ratio will be a close approximation to the risk ratio computed from the same follow-up data. However, in case-control studies, a risk ratio estimate cannot be computed, and an exposure odds ratio must be used instead. So, for casecontrol data, if the disease is "rare", does the exposure odds ratio approximate the risk ratio that would have resulted from a comparable follow-up study? The answer is yes, depending on certain conditions that must be satisfied, as we will now describe. This two-way table categorizes lung cancer and smoking status for a cohort of physicians in a large metropolitan city that are followed for 7 years. Forty smokers and twenty non-smokers developed lung cancer. The risk ratio is 2. Also, for this population, the risk odds ratio is equal to 2.02, essentially the same as the risk ratio. Since these are measures of effect for a population, we have not put the hat symbol over the risk ratio and risk odds ratio terms. We now consider the results that we would expect to obtain if we carried out a case-control study using this cohort as our source population. We will assume that the 7-year follow-up has occurred. We also assume that there exists a comprehensive cancer registry, so that we were able to find al1 60 incident cases that developed over the 7year period. These would be our cases in our case-control study. Now suppose we randomly select 60 controls from the source population as our comparison group. Since half of the entire cohort of 4000 physicians was exposed and half was unexposed, we would

84 Chapter 5. What’s the Answer? Measures of Effect expect 30 exposed and 30 unexposed out of the 60 controls. We can use the cross product ratio formula to compute the expected exposure odds ratio, which turns out to be 2. This value for the exposure odds ratio obtained from case-control data is the same that we would have obtained from the risk ratio and the risk odds ratio if we had carried out the follow-up study on this population cohort. In other words, the expected EOR from this case-control study would closely approximate the RR from a corresponding follow-up study, even if the follow-up study was never done! We may wonder whether the EOR would approximate the RR even if the 60 controls did not split equally into exposed and unexposed groups as expected. This can occur by chance from random selection or if we do a poor job of picking controls. For example, suppose there were 40 exposed and 20 unexposed among the controls. Then the estimated exposure odds ratio would equal 1 instead of 2, so in this situation, the EOR would be quite different from the RR obtained from a comparable follow-up study. What we have shown by example actually reflects an important caveat when applying the rare disease assumption to case-control data. The choice of controls in a case-control study must be representative of the source population from which the cases developed. If not, either by chance or a poor choice of controls, then the exposure odds ratio will not necessarily approximate the risk ratio even if the disease is rare. There is another important caveat for applying the rare disease assumption in a case-control study. The cases must be incident cases, that is, the cases need to include all new cases that developed over the time-period considered for determining exposure status. If the cases consisted only of prevalent cases at the time of case-ascertainment, then a biased estimate may result because the measure of effect would be estimating prevalence rather than incidence. Summary In case-control studies, the EOR approximates an RR when the following 3 conditions are satisfied: 1) The rare disease assumption holds 2) The choice of controls in the case-control study must be representative of the source population from which the cases developed. 3) The cases must be incident cases. Quiz (Q5.12): Understanding Risk Ratio In a case-control study, if the rare disease assumption is satisfied, then: 1. The ??? approximates the ??? provided that there is no ??? in the selection of ???, and the cases are ??? rather than ??? cases. Choices EOR ROR RR incidence prevalent bias cases controls randomness

A Pocket Guide to Epidemiology 85 In a community of 1 million persons, 100 cases of a disease were reported, distributed by exposure according to the table below. 2. Calculate the RR. 3. Calculate the ROR 4. Is this a rare disease? Ill Well Total Exposed 90 499,910 500,000 Non-Exposed 10 499,990 500,000 Total 100 999,900 1,000,000 ??? ??? ???

If the exposure status of all one million persons in the study population had not been available, the investigator may have conducted a case-control study. Suppose a random sample of 100 controls were selected. 5. Approximately what percentage of these controls would you expect to be exposed? ??? 6. What is the expected EOR in the case-control study? ??? Choices 0.11 10 50 9.00 90 no yes

Note: On Lesson Page 5-3 of the ActivEpi CD-ROM, there is an activity (and corresponding asterisk) that provides a mathematical proof of the odds ratio approximation to the risk ratio in case control studies. This proof makes use of conditional probability statements and Bayes Theorem.

The Rate Ratio
A rate ratio is a ratio of two average rates. It is sometimes called an incidence density ratio or a hazard ratio. Recall the general formula for an average rate: I denotes the number of new cases of the health outcome, and PT denotes the accumulation of person-time over the follow-up. The general data layout for computing a rate ratio is shown below. I1 and I0 denote the number of new cases in the exposed and unexposed groups, and PTl and PT0 denote the corresponding person time accumulation for these two groups. The formula for the rate ratio or the incidence density ratio (IDR) is also provided. We have used the notation IDR instead of RR to denote the rate ratio in order to avoid confusion with our previous use of RR to denote the risk ratio. As with both the risk ratio and odds ratio measures, the rate ratio can be >1, 1, 3.5), but the study data show essentially no effect, does this indicate a bias? Explain briefly. 5. If the true effect estimated indicates no association, but the study data show a high association, does this indicate a bias? Explain briefly. Summary Bias measures the extent that study results are distorted from the correct results that would have been found from a valid study. Bias occurs if an estimated measure of effect for the study population differs systematically from the value of the target parameter. Bias can rarely be quantified precisely primarily because the target parameter is always unknown. (“theta”) is the target parameter, is the sample estimate.
0

is the study population parameter, and
0

ˆ

ˆ is a biased estimate of provided Bias ( ˆ , ) = 0 -

is not equal to .

Relating the Target Parameter to Type of Bias The target parameter in most epidemiologic studies is typically a measure of effect, e.g., some kind of risk ratio, odds ratio, or rate ratio, appropriate for the study design being considered. The choice of parameter depends on whether the type of bias of interest is selection bias, information bias, or confounding bias, or whether more than one of these three types of bias is of concern. When selection bias is the only type of bias being considered, the target parameter is typically the value of the measure of effect of interest in the source population from which the cases are derived. If, however, information bias is the only type of bias considered, then the target parameter is the measure of effect that corrects for possible misclassification or that would result from the absence of misclassification. If there is only bias due to confounding, then the target parameter is the measure of effect estimated when confounding is controlled. If more than one bias is possible, then the target parameter is the value of the measure of effect after all contributing sources of bias are corrected.

A Pocket Guide to Epidemiology 119

Direction of the Bias
Although the precise magnitude of bias can never really be quantified, the direction of bias can often be determined. The direction of the bias concerns whether or not the target parameters is either overestimated or underestimated without specifying the magnitude of the bias. We have defined bias as the difference between the value of the effect measure in our target (i.e., source) population and the value of the effect measure actually being estimated in the study population. Since the target parameter is always unknown and the effect being estimated in the study population has random error, it is virtually impossible to quantify the magnitude of a bias precisely in a given epidemiologic study. Nevertheless, the investigator can often determine the direction of the bias. Such assessment usually requires subjective judgment based on the investigator 's knowledge of the variables being studied and the features of the study design that are the sources of possible bias. By direction, we mean a determination of whether the target parameter is overestimated or underestimated without specifying the magnitude of the bias. If the target parameter is overestimated, we say that the direction of the bias is away from the null. If the target parameter is underestimated, we say that the direction of the bias is towards the null. For example, suppose the target parameter is a risk ratio whose value is 1.5, but the risk ratio actually being estimated from our study is 4. Then the true effect has been overestimated, since the effect from the study appears to be stronger than it really is. Both the target parameter and the study population parameter in this example are greater than the null value of 1 for a risk ratio. Thus, the bias is away from the null, since the incorrect value of 4 is further away from 1 than the correct value of 1.5. Similarly, if the target risk ratio is less than the null value of 1, say .70 and the risk ratio being estimated is .25, the true effect is also overestimated. In this case, the true effect is protective, since it is less than 1, and the estimated effect of .25 is even more protective than the true effect. Again, the incorrect value of .25 is further away from 1 than the correct value of .70, so the bias is away from the null. To describe underestimation, or bias towards the null, suppose the target risk ratio is 4, but the estimated risk ratio is 1.5. Then the true effect is underestimated, since the effect from the study appears to be weaker than it really is. Moreover, the incorrect value of 1.5 in this case is closer to the null value of 1 than is the correct value of 4, so the bias is

120 Chapter 7. Is There Something Wrong? Validity and Bias towards the null. If the target risk ratio is 0.25 but the risk ratio being estimated is 0.70, then once again the correct value is underestimated and the bias is towards the null. In this case, the incorrect value of 0.70 is closer to the null value of 1 than is the correct value of .25. Suppose, however, that the target risk ratio is .50 but the risk ratio actually being estimated is 2. These two values are on opposite sides of the null value, so we cannot argue that the bias is either towards or away from the null. In this case, we call this kind of bias a switchover bias. In other words, a switchover bias may occur if the exposure appears in the data to have a harmful effect on the disease when it is truly protective. Alternatively, a switchover bias can occur if the exposure appears to be protective when it is truly harmful. Study Questions (Q7.6) Knowing the direction of the bias can be of practical importance to the investigator. 1. Suppose an investigator finds a very strong effect, say the estimated RR (in the study population) is 6, and she can also persuasively argue that any possible bias must be towards the null. Then what can be concluded about the correct (i.e., target) value of RR? 2. Suppose an investigator finds a very weak estimated RR (in the study population) of, say, 1.3, and can argue that any bias must be away from the null. Then what can be concluded about the correct value of the RR? Summary The direction of the bias concerns whether or not the target parameter is either overestimated or underestimated without specifying the magnitude of the bias. If the target parameter is overestimated, the direction of the bias is away from the null. If the target parameter is underestimated, the direction of the bias is towards the null. A switchover bias occurs if the target parameter is on the opposite side of the null value from the parameter actually being estimated in one’s study. Quiz (Q7.7) Label each of the following statements as True or False. 1. If the estimated RR equals 2.7 in the sample and it is determined that there is a bias away from the null, then the RR in the target (i.e., source) population is greater than 2.7. ??? 2. If the estimated RR in the sample is 1.1 and it is determined that there is bias towards the null, then there is essentially no association in the target population (as measured by RR). ???

A Pocket Guide to Epidemiology 121 3. If the estimated RR in the sample is 1.1 and it is determined that there is bias away from the null, then there is essentially no association in the target population. ??? 4. If the estimated RR equals 0.4 in the sample and it is determined that there is a bias away from the null, then the RR in the target (i.e., source) population is less than 0.4. ??? 5. If the estimated RR in the sample is 0.4 and it is determined that there is bias towards the null, then there is essentially no association in the target population (as measured by RR). ??? 6. If the estimated RR in the sample is 0.98 and it is determined that there is bias away from the null, then there is essentially no association in the target ??? population. Fill in the Blanks; Choices Away from the null Switchover Towards the null 7. If OR equals 3.6 in the target population and 1.3 in the study population, then the bias is . ??? 8. If IDR is 0.25 in the target population and 0.95 in the study population, then the bias is ??? 9. If the RR is 1 in the target population and 4.1 in the study population, then the bias is ??? 10. If the RR is 0.6 in the target population and 2.1 in the study population, then the bias is ??? 11. If the RR is 1 in the target population and 0.77 in the study population, then the bias is ??? 12. If the RR is 4.0 in the target population and 0.9 in the study population, then the bias is ???

What Can be Done About Bias?
The evaluation of bias is typically subjective and involves a judgment about either the presence of the bias, the direction of the bias, or, much more rarely, the magnitude of the bias. Nevertheless, there are ways to address the problem of bias, including adjusting the sample estimate to “correct” for bias. Three general approaches are now described. Here are three general approaches for addressing bias: 1) a priori study design decisions; 2) decisions during the analysis stage; and 3) discussion during the publication stage. When you design a study, you can make decisions to minimize or even avoid bias in the study 's results. You can avoid selection bias by including or excluding eligible subjects, by choice of the source population, or by the choice of the comparison group, say the control group in a case-control study.

122 Chapter 7. Is There Something Wrong? Validity and Bias Study Questions (Q7.8) 1. What type of bias may be avoided by taking special care to accurately measure the exposure, disease, and control variables being studied, including using pilot studies to identify measurement problems that can be corrected in the main study? A. Selection bias B. Information bias C. Confounding bias 2. What type of bias may be avoided by making sure to measure or observe variables at the design stage that may be accounted for at the analysis stage? B. Selection bias B. Information bias C. Confounding bias At the analysis stage, the investigator may be able to determine either the presence or direction of possible bias by logical reasoning about methodologic features of the study design actually used. Study Questions (Q7.8) continued 3. In the Sydney Beach User’s study, both swimming status and illness outcome were determined by subject self-report and recall. This indicates the need to assess the presence or direction of which type of bias at the analysis stage? A. Selection bias B. Information bias C. Confounding bias 4. Also, in the Sydney Beach Users study, subjects had to be excluded from the analysis if they did not complete the follow-up interview. This non-response may affect how representative the sample is. This is an example of which type of bias? A. Selection bias B. Information bias C. Confounding bias At the analysis stage, bias can also be reduced or eliminated by adjusting a sample estimate by a guestimate of the amount of bias. Such adjustment is typically done for confounding by quantitatively accounting for the effects of confounding variables using stratified analysis or mathematical modeling methods. Adjustment for selection bias and information bias is limited by the availability of information necessary to measure the extent of the bias. A simple formula for a "corrected" estimate involves manipulating the equation for bias by moving the target parameter to the left side of the equation. This formula is not as easy to apply as it appears. Most investigators will have to be satisfied with making a case for the direction of the bias instead. The estimated bias depends on the availability of more fundamental parameters, which are often difficult to determine. We discuss these parameters further in the chapters that follow. The final approach to addressing bias is how you report your study. A description of the potential biases of the study is typically provided in the "Discussion" section of a publication. This discussion, particularly when it

A Pocket Guide to Epidemiology 123 concerns possible selection or information bias, is quite subjective, but judgment is expected because of the inherent difficulty in quantifying biases. Rarely if ever does the investigator admit in the write-up that bias casts severe doubt on the study 's conclusions. So, the reader must review this section with great care! Summary – What Can be Done about Bias? The answer depends on the type of bias being considered: selection, information, or confounding. Approaches for addressing bias are: decisions in the study design stage, the analysis stage, and the publication stage. At the study design stage, steps can be taken to avoid bias. At the analysis stage, one may use logical reasoning about methodologic features of the study design actually used. Also at the analysis stage, confounding bias can be reduced or eliminated by quantitatively adjusting the sample estimate. A simple formula for a corrected estimate: c ˆ Bias . This formula is not as ˆ easy to apply as it looks. Potential biases are described in the Discussion section of a publication. Beware!

Nomenclature

ˆ
0

“theta”, the parameter from the target population “theta-hat”, the parameter estimate from the sample actually analyzed The parameter from the study population Risk ratio of the target population Risk ratio of the study population

RR RR0

References
Berkson J. Limitations of the application of fourfold table analysis to hospital data. Biometrics Bulletin 1946;2: 47-53. Corbett SJ, Rubin GL, Curry GK, Kleinbaum DG. The health effects of swimming at Sydney Beaches. The Sydney Beach Users Study Advisory Group. Am J Public Health. 1993;83(12): 1701-6. Greenberg RS, Daniels SR, Flanders WD, Eley JW, Boring JR. Medical Epidemiology (3rd Ed). Lange Medical Books, New York, 2001. Hill H, Kleinbaum DG. Bias in Observational Studies. In Encyclopedia of Biostatistics, pp 323-329, Oxford University Press, 1999. Horwitz RI, Feinstein AR. Alternative analytic methods for case-control studies of estrogens and endometrial cancer. N Engl J Med 1978;299(20):1089-94. Kleinbaum DG, Kupper LL, Morgenstern H. Epidemiologic Research: Principles and Quantitative Methods. John Wiley and Sons Publishers, New York, 1982. Perera FP, et al. Biologic markers in risk assessment for environmental carcinogens. Environ Health Persp 1991;90:247. The Lipid Research Clinics Coronary Primary Prevention Trial results: II. The relationship of coronary heart disease to cholesterol lowering. JAMA 1984;251(2):365-74. Warner L, Clay-Warner J, Boles J, Williamson J. Assessing condom use practices. Implications for evaluating method and user effectiveness. Sex Transm Dis 1998; 25(6):273-7.

124 Chapter 7. Is There Something Wrong? Validity and Bias

Answers to Study Questions and Quizzes
Q7.1 1. Women residents from Atlanta between 45 and 69 years of age. 2. A measure of effect, either a prevalence ratio or a prevalence odds ratio (i.e., the blue dot or bulls eye), for the association between calcium intake and prevalence of arthritis of the hip in the target population. 3. Random error concerns whether or not the estimated odds ratio in the hospital sample (i.e., the shot at the target) differs from the odds ratio in the population of hospital patients (i.e., the red dot or the center of the actual shots) from which the sample is selected. 4. Systematic error concerns whether or not the odds ratio in the population of hospital patients being sampled (i.e., the red dot) is different from the odds ratio in the target population (i.e., the blue dot). Q7.2 1. All young adults in New York City. 2. All young adults who attend fitness centers in New York City (NYC) and would eventually remain in the study for analysis. 3. Yes, the sample is randomly selected from the study population and is therefore representative. 4. Probably not. The group of young adults in NYC is different from the group of all young adults in NYC. Since fitness is so strongly related to health, the use of those attending fitness centers for all young adults is probably not the best choice for this study. 5. All young adults in New York City. 6. All young adults in NYC that would eventually remain in the study for analysis. 7. Yes, the sample is randomly selected from the study population (by definition of the study population) and is therefore representative of it. Nevertheless, neither the study population nor the sample may be representative of all young adults in NYC if not everyone selected into the sample participates in the study.

8. Yes, assuming that everyone selected participates (i.e., provides the required data) in the study, the study population is the same as the source population. However, if many of those sampled (e.g., a particular subgroup) do not participate, the final sample and its corresponding study population might be unrepresentative of all young adults in NYC. 9. False. Persons lost-to-follow-up are not found in the study sample, so they can’t be included in the study population that is represented by the sample. 10. True. Persons lost-to-follow-up are not found in the sample, but they are still included in the source population of interest. 11. True. In the study population, controls may be specified as persons without the disease, regardless of whether they are at risk for being a case. However, the source population may only contain persons at risk for being a case. 12. False. The study population in a crosssectional study is restricted to survivors only. 13. The population of young adults in New York State is an external population, because the study was restricted to young adults in New York City. Extrapolating the study results to New York State goes beyond considering the methodological aspects of the actual study. Q7.3 1. The study will be internally valid provided the study population corresponding to the sample actually analyzed is not substantially distorted from the source population of young adults from fitness centers in the city. For example, if the sample eventually analyzed is a much healthier population than the source population, internal validity may be questioned. 2. External validity. The study was restricted to persons in New York City. Extrapolating the study results to New York State goes beyond considering the methodological aspects of the New York City study

A Pocket Guide to Epidemiology 125
3. Do the results of the study also apply to women or to men of different ages? 4. Was the study sample representative of the source population? Were the comparison groups selected properly? Did subjects consistently stick to their diet and medication regimen? Were relevant variables taken into account? Q7.4 1. T 2. F – Systematic error occurs when there is a difference between the true effect measure and that which is actually estimated, i.e., a difference between the source and study populations. 3. F – A valid study means there is no systematic error, but there may still be random error 4. F – Not always. Ideally the study population would be equivalent to the source population. However, it may be that the study population and source population simply overlap. 5. T – The sample is always selected from the study population. 6. F – If the study population is not a subset of or equivalent to the source population, then the sample may not be a subset of (or completely contained in) the source population. 7. F – They may be different due to random error. 8. It depends – The difference may be a result of random error. If the risk ratio in the study population is meaningfully different from 3.0, then the study is not internally valid. 9. T – Any meaningful difference between the study and source population means that the study is not internally valid. 10. F – The difference between 3.0 and 3.1 would not be considered a meaningful difference. 11. It depends – We do not know about the risk ratio in the external population. Q7.5 1. The target population parameter is 3.5. We don’t know the value of the study population parameter, but it is likely to be closer to 1.5 than to 3.5 because the sample is assumed to be representative of the study population. 2. There appears to be bias in the study because the sample estimate of 1.5 is meaningfully different from the population estimate and it is reasonable to think that the final sample no longer represents the source population. 3. The bias can’t be determined exactly because 1.5 is a sample estimate; however, the bias in the risk ratio is approximately 1.5 – 3.5 = -2. 4. Yes, provided the reason for the difference is due to systematic error. 5. Yes, provided the reason for the difference is due to systematic error. Q7.6 1. The correct RR must be even larger than 6. 2. The correct RR must indicate an even weaker, or no effect. Q7.7 1. F – If the bias is away from the null, the RR in the source population must lie between 1 and 2.7. 2. F – If the bias is towards the null, then the RR in the source population is greater than 1.1. Since we cannot determine how much greater, we cannot conclude that these is essentially no association. 3. T – If the bias is away from the null, then the RR in the source population is between 1 and 1.1. We can thus conclude that there is essentially no association. 4. F – If the bias is away from the null, then the RR in the source population must lies between 0.4 and 1.0. 5. F – If the bias is towards the null, then the RR in the source population must be less than 0.4 and hence there is an association. 6. T – If the bias is away from the null, then the RR in the source population is between 0.98 and 1.0, which means there is essentially no association. 7. Towards the null 8. Towards the null 9. Away from the null 10. Switchover 11. Away from the null 12. Switchover Q7.8 1. B; 2. C; 3. B; 4. A

CHAPTER

8

WERE SUBJECTS CHOSEN BADLY? SELECTION BIAS
Selection bias concerns systematic error that may arise from the manner in which subjects are selected into one’s study. In his chapter we describe examples of selection bias, provide a quantitative framework for assessing selection bias, show how selection bias can occur in different types of epidemiologic study designs, and discuss how to adjust for or otherwise deal with selection bias.

Selection Bias in Different Study Designs
Selection bias is systematic error that results from the way subjects are selected into the study or because there are selective losses of subjects prior to data analysis. Selection bias can occur in any kind of epidemiologic study. In casecontrol studies, the primary source of selection bias is the manner in which cases, controls, or both are selected and the extent to which exposure history influences such selection. For example, selection bias was of concern in casecontrol studies that found an association between use of the supplement Ltryptophan and EMS (eosinophilia myalgia syndrome), an illness characterized primarily by incapacitating muscle pains, malaise, and elevated eosinophil counts. The odds ratios obtained from these studies might have overestimated the true effect. Study Questions (Q8.1) 1. Assuming that the odds ratio relating L-tryptophan to EMS is overestimated, which of the following choices is correct? a. The bias is towards the null. b. L-tryptophan has a weaker association with EMS than actually observed. c. The correct odds ratio is larger than the observed odds ratio. Consequently, the bias would be away from the null. A primary criticism of these studies was that initial publicity about a suspected association may have resulted in preferential diagnosis of EMS among known users of L-tryptophan when compared with nonusers. Study Questions (Q8.1) continued 2. Assuming preferential diagnosis of EMS from publicity about L-tryptophan, which of the following is correct? The proportion exposed among diagnosed cases selected for study is likely to be ??? the proportion exposed among all cases in the source population. a. larger than b. smaller than c. equal to In cohort studies and clinical trials, the primary sources of selection bias are

128 Chapter 8. Were Subjects Chosen Badly? Selection Bias loss-to-follow-up, withdrawal from the study, or non-response. For example, consider a clinical trial that compares the effects of a new treatment regimen with a standard regimen for a certain cancer. Suppose patients assigned to the new treatment are more likely than those on the standard to develop side effects and consequently withdraw from the study. Study Questions (Q8.2) Clinical trial: new cancer regimen versus standard regimen. Suppose patients on new regimen are more likely to withdraw from study than those on standard. 1. Why might the withdrawal information above suggest the possibility of selection bias in this study? 2. Why won’t and intention-to-treat analysis solve this problem? 3. What is the source population in this study? 4. What is the study population in this study? In cross-sectional studies, the primary source of selection bias is what is called selective survival. Only survivors can be included in cross-sectional studies. If exposed cases are more likely to survive longer than unexposed cases, or vice versa, the conclusions obtained from a cross-sectional study might be different than from an appropriate cohort study. Study Questions (Q8.2) continued Suppose we wish to assess whether there is selective survival in a cross-sectional study. 5. What is the source population? 6. What is the study population? Summary Selection bias can occur from systematic error that results from the way subjects are selected into the study and remain for analysis. The primary reason for such bias usually differs with the type of study used. In case-control studies, the primary source of selection bias is the manner in which cases, controls, or both are selected. In cohort studies and clinical trials, the primary source of selection bias is loss to follow-up, withdrawal from the study, or non-response. In cross-sectional studies, the primary source of selection bias is what is called selective survival.

Example of Selection Bias in Case-Control Studies
In case-control studies, because the health outcome has already occurred, the selection of cases, controls, or both might be influenced by prior exposure status. In the 1970 's, there was a lively published debate about selection bias in studies of whether use of estrogen as a hormone replacement leads to endometrial cancer. Early studies were case-control studies and they indicated a strong harmful effect of estrogen. The controls typically used were women with gynecological cancers other than endometrial cancer. Critics claimed that because estrogen often causes vaginal bleeding irrespective of cancer, estrogen users with endometrial cancer would be selectively screened for such cancer compared to nonusers with endometrial cancer.

A Pocket Guide to Epidemiology 129 Study Questions (Q8.3) 1. If the critics reasoning were correct, why would there be a selection bias problem with choosing controls to be women with gynecological cancers other than endometrial cancer? 2. If the critics reasoning were correct, would you expect the estimated odds ratio obtained from the study to be biased towards or away from the null? Explain briefly. An alternative choice of controls was proposed; women with benign endometrial tumors, since it was postulated that such a control group would be just as likely to be selectively screened as would the cases. Study Questions (Q8.3) continued 3. Why would estrogen users with benign endometrial tumors be more likely to be selectively screened for their tumors when compared to nonusers? 4. Assuming that estrogen users with both cancerous and benign endometrial tumors are likely to be selectively screened for their tumors when compared to non-users, what problem may still exist if the latter group is chosen as controls? Continued research and debate, however, have indicated that selective screening of cases is not likely to contribute much bias. In fact, the proposed alternative choice of controls might actually lead to bias. Study Questions (Q8.3) continued Researchers concluded that because nearly all women with invasive endometrial cancer will ultimately have the disease diagnosed, estrogen users will be slightly over-represented, if at all, among a series of women with endometrial cancer. Assume for the questions below that selective screening of cases does not influence the detection of endometrial cancer cases. 5. If the control group consisted of women with benign tumors in the endometrium, why would you expect to have selection bias? 6. Would the direction of the bias be towards or away from the null? Briefly explain. 7. If the control group consisted of women with gynecologic cancers other than in the endometrium, why would you not expect selection bias in the estimation of the odds ratio? In current medical practice, the prevailing viewpoint is that taking estrogen alone is potentially harmful for endometrial cancer. Consequently, women who are recommended for hormone replacement therapy are typically given a combination of progesterone and estrogen rather than estrogen alone. Summary Selection bias concerns a distortion of study results that occurs because of the way subjects are selected into the study. In case-control studies, the primary concern is that selection of cases, controls, or both might be influenced by prior exposure status. In the 1970’s, there was a lively published debate about possible selection bias

130 Chapter 8. Were Subjects Chosen Badly? Selection Bias among researchers studying whether use of estrogen, the exposure, as a hormone replacement leads to endometrial cancer. The argument supporting selection bias has not held up over time; current medical practice for hormone replacement therapy typically involves a combination of progesterone and estrogen rather than estrogen alone.

Example of Selection Bias in Cohort Studies
Selection bias can occur in cohort studies as well as in case-control studies. In prospective cohort studies, the health outcome, which has not yet occurred when exposure status is determined, cannot influence how subjects are selected into the study. However, if the health outcome is not determined for everyone initially selected for study, the study results may be biased. The primary sources of such selection bias are loss-to-follow-up, withdrawal or non-response. The collection of subjects that remain to be analyzed may no longer represent the source population from which the original sample was selected. Consider this two-way table that describes the five-year follow-up for disease “D” in a certain source population. Suppose that a cohort study is carried out using a 10% random sample from this population. Consider the following table of expected cell frequencies for this cohort: Study Questions (Q8.4) 1. Assuming this is the sample that is analyzed, is there selection bias? Assume that the initial cohort was obtained from the 10% sampling. However, now suppose that 20% of exposed persons are lost to follow-up but 10% of unexposed persons are lost. Also, assume that exposed persons have the same risk for disease in the final cohort as in the initial cohort and that the same is true for unexposed persons. Study Questions (Q8.4) continued 2. Does the sample just described represent the source population or the study population? 3. What is the source population for which this sample is derived? 4. For the above assumptions, is there selection bias? Suppose that a different pattern of loss-to follow-up results in the two-way table shown here.

A Pocket Guide to Epidemiology 131 Study Questions (Q8.4) continued 5. Do these results indicate selection bias? 6. Do the exposed persons in the study population have the same risk for disease as in the source population? 7. Do the unexposed persons in the study population have the same risk for disease as in the source population? 8. Do the previous examples demonstrate that there will be selection bias in cohort studies whenever the percent lost to follow-up in the exposed group differs from the percent lost-to-follow-up in the unexposed group? Summary The primary sources of selection bias in cohort studies are loss-to-follow-up, withdrawal, and non-response. In cohort studies, the collection of subjects that remain in the final sample that is analyzed may no longer represent the source population from which the original cohort was selected. Selection bias will occur if loss to follow-up results in risk for disease in the exposed and/or unexposed groups that are different in the final sample than in the original cohort.
Some Fine Points about Selection Bias in Cohort Studies Reference: Hill, HA and Kleinbaum, DG, "Bias in Observational Studies", in the Encyclopedia of Biostatistics, P.A. Armitage and T Colton, eds., June 1998. Selection bias in cohort studies may occur even with a fairly high overall response rate or with very little loss to follow-up. Consider a cohort study in which 95% of all subjects originally assembled into the cohort remain for analysis at the end of the study. That is, only 5% of subjects are lost to follow-up. If losses to follow-up are primarily found in exposed subjects who develop the disease, then despite the small amount of follow-up loss, the correct (i.e., target) risk ratio could be underestimated substantially. This is because, in the sample that is analyzed, the estimated risk for developing the disease in exposed subjects will be less than what it is in the source population, whereas the corresponding risk for unexposed subjects will accurately reflect the source population. There may be no selection bias despite small response rates or high loss to follow-up. Suppose only 10% of all initially selected subjects agree to participate in a study, but this 10% represents a true random sample of the source population. Then the resulting risk ratio estimate will be unbiased. The key issue here is whether risks for exposed and unexposed in the sample that is analyzed are disproportionately modified because of non-response or follow-up loss from the corresponding risks in the source population from which the initial sample was selected. We are essentially comparing two 2x2 tables here, one representing the source population and the other representing the sample: Source Population E Not E A B C D N1 N0 Sample for Analysis E Not E a b c d n1 n0

D Not D Total

D Not D Total

Selection bias will occur only if, when considering these tables, the risk ratio in the source population, i.e.,

132 Chapter 8. Were Subjects Chosen Badly? Selection Bias
A/N1 is meaningfully different from the risk ratio in the sample, i.e., a/n1 B/N 0 b/n 0 In the first example above (95% loss to follow-up), the argument for selection bias is essentially that the numerator a/n1 of the risk ratio in the analyzed sample would be less than the corresponding numerator A/N1 in the source population, whereas the corresponding denominators in these two risk ratios would be equal. In the second example (10% non-response), the argument for no selection bias is essentially that despite the high non-response, corresponding numerators and denominators in the source population and sample are equal.

Other Examples
Here are a few more examples of studies that are likely to raise questions about selection bias. Study Questions (Q8.5) Consider a retrospective cohort study that compares workers in a certain chemical industry to a population-based comparison group for the development of coronary heart disease (CHD). 1. In such a study, selection bias may occur because of the so-called “healthy worker effect”. How might such a bias come about? Selection bias may result from using volunteers for a study. 2. Explain the above statement in terms of study and source populations. 3. What is an alternative way to view the validity problem that arises when a study is restricted to volunteers? In assessing long term neurologic disorders among children with febrile seizures, clinic-based studies tend to report a much higher frequency of such disorders than found in population-based studies. 4. Does the above statement indicate that clinic-based studies can result in selection bias? Explain briefly. Summary Selection bias may occur because of the so-called “healthy worker effect”. Workers tend to be healthier than those in the general population and may therefore have a more favorable outcome regardless of exposure status. Selection bias may result from using volunteers, who may have different characteristics from persons who do not volunteer. Clinic-based studies may lead to selection bias because patients from clinics tend to have more severe illness than persons in a population-based sample.

What Can Be Done About Selection Bias Qualitatively?
A description of quantitative formulas for correcting for selection bias, see is provided on pages 8-2 and 8-5 of the ActivEpi CD. Unfortunately the information required to apply these formulas, namely the selection ratio parameters or their ratios, is conceptually complicated, rarely available, and not easily quantified. The ideal way to address selection bias is to prevent or at least minimize such bias when designing a study rather than to attempt to correct for the

A Pocket Guide to Epidemiology 133 bias once the data have been collected. In case-control studies, the controls should be carefully chosen to represent the source population that produced the cases. The use of two or more control groups should also be considered. Case-control studies using incident cases and nested case-control studies are preferable to studies that use prevalent cases or to hospital-based studies. Selection bias may also be avoided by assuring equal opportunity for disease detection among exposed and unexposed subjects. In cohort studies, efforts should be made to achieve high response and low loss-to-follow-up. Observational studies involving volunteers should be avoided, although clinical trials involving volunteers are possible because of randomization. Occupational cohort studies should avoid a healthy worker bias by ensuring that unexposed subjects are as healthy as exposed subjects. At the analysis stage, it may be possible to determine the direction of the bias, even if the magnitude of the bias cannot be estimated or a numerical correction for bias is not feasible. One approach to addressing selection bias as well as information and confounding biases is in the write-up of the study. A description of the potential biases of the study is typically provided in the "Discussion" section of a publication. This discussion, particularly when it concerns possible selection bias, is quite subjective, but such judgment is required because of the inherent difficulty in quantifying such biases. Summary The information required to assess selection bias is conceptually complicated, rarely available, nor is easily quantifiable. At the study design stage, decisions may be made to avoid selection bias in the study’s results. At the analysis stage, it may be possible to determine the direction of selection bias without being able to quantitatively correct for the bias. At the publication stage, potential biases are typically addressed qualitatively in the Discussion section of a paper.
The "Worst-Case Scenario" Approach This is a practical approach for assessing the direction of selection bias that considers the most extreme changes in the estimate of effect that are realistically possible as a result of the way subjects are selected. Through such an approach it may be possible to show that the worst amount of bias possible will have a negligible effect on the conclusions of one 's study. For example, consider a cohort study involving lung-cancer-free 1000 smokers and 1000 non-smokers all over 40 years of age that are followed for 10 years. Suppose further that over the follow-up period, 200 smokers and 100 non-smokers are lost-to-follow-up. Also, suppose, that among those 800 smokers and 900 non-smokers remaining in the study, the 2x2 table relating smoking status at the start of the study to the development of lung cancer (LC) over the 10 year follow-up is shown as follows: Smokers Non-smokers 80 10 LC 720 890 No LC 800 900 Total

134 Chapter 8. Were Subjects Chosen Badly? Selection Bias
The estimated risk ratio from these data is (80/800)/(10/900) = 9, which suggests a very strong relationship between smoking status and the development of lung cancer. A worstcase scenario might determine what the risk ratio estimates would be if either all 200 smokers lost-to-follow-up did not develop lung cancer and/or all 100 non-smokers lost to follow-up did develop lung. Here are comparison of estimates for “worst-case” scenarios: Scenario RR 1. Actual observed data 9 2. 1/10th of 200 lost-to-follow-up smokers get LC and 1/90th of the 100 lost-to9 follow-up non-smokers get LC 3. 1/10th of the 200 lost-to-follow-up smokers get LC and 2/90th of the 100 lost- 8.2 to-follow-up non-smokers get LC 4. None of the 200 lost-to-follow-up smokers get LC and 1/90th of the 100 lost- 7.3 to-follow-up non-smokers get LC 5. None of the 200 lost-to-follow-up smokers get LC and 2/90th of the 100 lost- 6.6 to-follow-up non-smokers get LC 6. None of the 200 lost-to-follow-up smokers get LC and all 100 lost-to-follow- 0.7 up non-smokers get LC Notice that scenario #6 above changes smoking from being harmful to being protective. Yet this is not a very realistic scenario. Scenario #2 is not really a "worst case" type of scenario because it assumes that those lost to follow-up have the same risk as those actually followed over the entire 10 years. The other three scenarios, i.e., # 's 3, 4, and 5, are "realistic" and of a "worst-case" type; all of these show that the risk ratio is reduced, but is still high. The difficulty with this approach, as illustrated above, concerns the extent to which the investigator can identify a "worst-case" scenario that is the most realistic among all possible scenarios.

Quiz (Q8.6) 1. Case-control studies using ??? are preferable to studies that use ???. 2. And ??? studies are preferable to ??? studies. 3. In cohort studies, efforts should be made to achieve ??? and to avoid ???. Choices cases controls high response hospital-based incident cases loss-to-follow-up nested case-control prevalent cases selection bias At the analysis stage, the extent of possible selection bias may be assessed using what are often referred to as "worst-case" analyses. Such analyses consider the most extreme changes in the estimated effect that are possible as a result of selection bias. Determine whether each of the following is True or False. 4. This approach is useful since it could demonstrate that the worst amount of bias possible will have a negligible effect on the conclusions of the study. ??? 5. This approach can rule out selection bias, but it cannot confirm selection bias. ??? The tables below show the observed results and a 'worst-case ' scenario for a clinical trial. Ten subjects receiving standard treatment and 15 subjects receiving a new treatment were lost-to-follow-up. The outcome was whether or not a subject went out of remission (D = out, not D = in) by the end of the trial. In the 'worstcase ' scenario, all 10 subjects on standard treatment who were lost-to-follow-up

A Pocket Guide to Epidemiology 135 remained in remission but all 15 subjects on the new treatment who were lost-tofollow-up went out of remission.

6. What are the values of a*? ???, b*? ???, c*? ???, d*? ??? Choices 106 110 121 22 32 42 90 91

7. Refer to the data in the tables above. What is the risk ratio estimate based on the observed data? ??? 8. What is the risk ratio estimate based on the 'worst-case ' scenario? ??? 9. In the worst-case scenario, is there selection bias? ??? 10. Does a “worst-case” assessment such as illustrated here “prove” that there is selection bias? ??? Choices 1.3 1.4 3.9 4.8 No Yes

References
References on Selection Bias Berkson J. Limitations of the application of fourfold table analysis to hospital data. Biometrics Bulletin 1946;2: 47-53. Greenberg RS, Daniels SR, Flanders WD, Eley JW, Boring JR. Medical Epidemiology (3rd Ed). Lange Medical Books, New York, 2001. Hill H, Kleinbaum DG. Bias in Observational Studies. In Encyclopedia of Biostatistics, pp 323-329, Oxford University Press, 1999. Horwitz RI, Feinstein AR. Alternative analytic methods for case-control studies of estrogens and endometrial cancer. N Engl J Med 1978;299(20):1089-94. Kleinbaum DG, Kupper LL, Morgenstern H. Epidemiologic Research: Principles and Quantitative Methods. John Wiley and Sons Publishers, New York, 1982. References on Condom Effectiveness Studies Warner L, Clay-Warner J, Boles J, Williamson J. Assessing condom use practices. Implications for evaluating method and user effectiveness. Sex Transm Dis 1998; 25(6):273-7. Warner DL, Hatcher RA. A meta-analysis of condom effectiveness in reducing sexually transmitted HIV. Soc Sci Med 1994;38(8):1169-70.

Answers to Study Questions and Quizzes
Q8.1 1. b 2. a; exposed cases are likely to be overrepresented in the study when compared to unexposed cases. Q8.2 1. Withdrawals from the study can distort the final sample to be analyzed as compared to the random sample obtained at the start of the trial. 2. Those who withdraw from the study have an unknown outcome and therefore cannot be analyzed. 3. A general population of patients with the specified cancer and eligible to

136 Chapter 8. Were Subjects Chosen Badly? Selection Bias receive either the standard or the new treatments. 4. The (expected) sample ignoring random error that would be obtained after the withdrawals from the random sample initially selected for the trial. 5. The source population is the population cohort from which the cases would be derived if an appropriate cohort study had been carried out. 6. The study population is the expected sample obtained from the crosssectional sample that is retained for analysis. Alternatively, the study population is the stable population from which the cross-sectional sample is obtained for study. Q8.3 1. It is unlikely that women in this control group (e.g., with cervical or ovarian cancers) would be selectively screened for their cancer from vaginal bleeding caused by estrogen use. 2. Away from the null because of selective screening of cases but not controls. This would yield too high a proportion of estrogen users among cases but a correct estimate of the proportion of estrogen users among controls. 3. Because those who have vaginal bleeding from using estrogen will be more likely to have their benign endometrial tumor detected than those non-users with benign endometrial tumors. 4. Using benign endometrial tumors as the control group would hopefully compensate for the selective screening of cases. However, it is not clear that the extent of selective screening would be the “same” for both cases and controls. 5. Having a benign tumor in the endometrium is not readily detected without vaginal bleeding. Therefore, controls with benign endometrial tumors who use estrogen are more likely to have their tumor detected than would nonuser controls. 6. Towards the null because there would be selective screening of controls but not cases. This would yield too high a proportion of estrogen users among controls but a correct estimate of the proportion of estrogen users among cases. 7. Because there is unlikely to be selective screening in the detection of control cases (with other gynecological cancers) when comparing estrogen users to nonusers. Q8.4 1. No, since the risk ratio for the expected sample is 3, which equals the risk ratio in the source population. 2. Study population, because the sample just described gives the expected number of subjects obtained in the final sample. 3. The source is the population from which the initial 10% sample obtained prior to follow-up was selected. This is the population of subjects from which cases were derived. 4. There is no selection bias because the RR=3 in the study population, the same as in the source population. 5. Yes, because the estimated risk ratio in the study population of 3.9 is somewhat higher than (3) in the source population as a result of subjects being lost to follow-up. 6. No, the risk for exposed persons is 150/10,000 or .0150 in the source population and is 14/800 = .0175 in the sample. 7. No, the risk for unexposed persons is 50/10,000 or .0050 in the source population and 4/900 or .0044 in the sample. 8. No. There will only be selection bias if loss to follow-up results in risks for disease in the exposed and/or unexposed groups that are different in the final sample than in the original cohort. Q8.5 1. Workers tend to be healthier than those in the general population and may therefore have a more favorable outcome regardless of exposure status. 2. Volunteers may have different characteristics from person who do not volunteer. The study population here is restricted to volunteers, whereas the

A Pocket Guide to Epidemiology 137 source population is population-based, e.g., a community. 3. There is lack of external validity in drawing conclusions from a source population of volunteers to an external population that is population-based. 4. Yes. Clinic-based studies may lead to spurious conclusions because patients from clinics tend to have more severe illness than persons in a populationbased sample. Q8.6 1. incident cases, prevalent cases 2. nested case-control, hospital-based 3. high response, loss-to-follow-up 4. True 5. True – Since the “worst-case” analysis can demonstrate that the worst amount of bias will have a negligible effect on the conclusions, one could rule out selection bias. However, since the “worst-case” analysis gives us the “worst possible” results, we cannot confirm selection bias. We cannot be sure that our results will be as extreme as “worst-case” results. 32, 22, 110, 106 3.9 1.3 Yes No - A worst-case analysis gives the “worst possible” results. Therefore, we cannot be sure that the lost-to-followup results that “actually” occur are as extreme as the worst-case “possible”.

6. 7. 8. 9. 10.

CHAPTER

9

ARE THE DATA CORRECT? INFORMATION BIAS
Information bias is a systematic error in a study that arises because of incorrect information obtained on one or more variables measured in the study. The focus here is on the consequences of having inaccurate information about exposure and disease variables that are dichotomous, that is, when there is misclassification of exposure and disease that leads to a bias in the resulting measure of effect. We consider exposure and disease variables that are dichotomous. More general situations, such as several categories of exposure or disease, continuous exposure or disease, adjusting for covariates, matched data, and mathematical modeling approaches, are beyond the scope of the activities provided below.

What is Misclassification Bias?
The two-way table to the right shows the correct classification of 16 subjects according to their true exposure and disease status. Let 's see what might happen to this table and its corresponding odds ratio if some of these subjects were misclassified. Suppose that 3 of the 6 exposed cases, shown here in the lighter shade, were actually misdiagnosed as exposed non-cases. Suppose further that one of the two unexposed cases was also misclassified as an unexposed non-case. To complete the misclassification picture, we assume that two of the four truly exposed noncases and two of the four truly unexposed non-cases were misclassified as cases. The misclassified data are the data that would actually be analyzed because these data are what is observed in the study. So, what is the odds ratio for these data and how does it compare to the correct odds ratio? The odds ratio for the misclassified data is 1; the correct odds ratio is 3. Clearly, there is a bias due to misclassification. The misclassified data suggests no effect of exposure on disease, but the true effect of exposure is quite strong. Summary If subjects are misclassified by exposure or disease status, the effect measure, e.g., the OR, may become biased Bias from misclassification can occur if the effect measure for the correctly classified data is meaningfully different from the estimated effect actually observed in the misclassified data.

140 Chapter 9. Are The Data Correct? Information Bias Subjects are misclassified if their location in one of the four cells of the correctly classified data changes to a different cell location in the (misclassified) data that is actually observed.

Misclassifying Disease Status
What are the reasons why a subject, like an exposed case, might be misclassified on exposure or disease status? In particular, why might subjects be misclassified from diseased to non-diseased or from nondiseased to diseased status? First, a subject may be incorrectly diagnosed. This can occur because of limited knowledge about the disease, because the diagnostic process is complex, because of inadequate access to state-of-the-art diagnostic technology, or because of a laboratory error in the measurement of biologic markers for the disease. In addition, the presence of disease may be not be detected if the disease is subclinical at the time of physical exam. Misdiagnosis can occur because of a detection bias if a physician gives a more thorough exam to patients who are exposed or have symptoms related to exposure. Another source of error occurs when disease status is obtained solely by selfreport of subjects rather than by physician examination. In particular, a subject may incorrectly recall illness status, such as respiratory or other infectious illness, that may have occurred at an earlier time period. A subject may be reluctant to be truthful about an illness he or she considers socially or personally unacceptable. Finally, patient records may be inaccurate or coded incorrectly in a database. The table to the right summarizes misclassification of disease status. The columns of the table show true disease status. The rows of the table show classified disease status. We call this table a misclassification table. Suppose the following numbers appear in the table:

A Pocket Guide to Epidemiology 141 Study Questions (Q9.1) The following questions refer to the previous table. 1. How many truly diseased persons are misclassified? 2. How many truly non-diseased persons are correctly classified? 3. The percentage of truly diseased persons correctly classified? 4. The percentage of truly non-diseased persons correctly classified? Summary Misclassification of disease status may occur from any of the following sources: Incorrect diagnosis, subject self-report, and coding errors A misclassification table provides a convenient summary of how disease status can be misclassified from true disease status to observed disease status.

Misclassifying Exposure Status
How can subjects be misclassified from exposed to unexposed or from unexposed to exposed? Misclassification of exposure status can occur because of imprecise measurement of exposure. This can result from a poorly constructed questionnaire or survey process that doesn 't ask the right questions, or from a faulty measuring device or observation technique. Study Questions (Q9.2) A primary criticism of studies evaluating whether living near power lines increases one’s risk for cancer is the quality of measurements of personal exposure to electromagnetic fields (EMFs). Which of the following “reasons” for imprecise measurement of personal EMF exposure do you think are True or False? 1. Measurements are usually made at only one time point and/or in one location of a residence. 2. Instruments for measuring EMF exposure are not available. 3. Methods for measuring distances and configuration of transmission lines near residences are poorly developed. 4. Better methods for monitoring measurements over time are needed. 5. Measuring EMF exposure from intermittent use of appliances or tools is difficult to measure. 6. Mobility patterns of individual related to EMF exposure are difficult to measure. Exposure error may occur when exposure is determined solely from self-report by subjects, particularly when recalling prior exposure status. This is typically a problem in case-control studies, since cases may be more motivated to recall past exposures than controls. Recall error can also occur in cohort studies. For example, in the Sydney Beach Users study described in Chapter 2, subjects were asked to report their swimming status seven days after swimming may have occurred. Subject self-report may also be incorrect because of reluctance of subjects to be truthful in reporting exposures relating to behaviors considered socially

142 Chapter 9. Are The Data Correct? Information Bias unacceptable. This problem often occurs in studies that measure food intake, sexual behavior, and illegal drug-use. A third source of error in classifying exposure is interviewer bias. In particular, an interviewer may probe more thoroughly about exposure for cases than for controls. Study Questions (Q9.2) continued Consider a case-control study of the effect of oral contraceptive use on the development of venous (i.e., in the vein) thrombosis (i.e., clotting). (Note: there are no questions numbered 7 to 9.) 10. Why might there be misclassification of exposure, i.e., oral contraceptive use, in such a study? 11. What you expect to be the direction of such misclassification bias? 12. How might you avoid such bias? Finally, exposure data can be coded incorrectly in a database. The table to the right summarizes exposure status misclassifications. The columns of the table show true exposure status. The rows of the table show classified exposure status. Study Questions (Q9.2) continued The questions are based on the table to the right. 13. How many truly exposed persons are misclassified? 14. How many truly unexposed persons are correctly classified? 15. The percentage of truly exposed persons correctly classified? 16. The percentage of truly unexposed persons correctly classified? Summary Misclassification of exposure status may occur from any of the following sources: Imprecise measurement, subject self-report, interviewer bias, and incorrect coding of exposure data A misclassification table provides a convenient summary of how exposure can be misclassified from true exposure to observed exposure.

Example: Misclassifying Both Exposure and Disease Status
Misclassification can sometimes occur for both exposure and disease in the same study. For example, the table to the right considers hypothetical cohort data from subjects surveyed on the beaches of Sydney, Australia during the summer months of a recent year. The study objective was to determine if those who swam at the beach were more likely to become ill than those who did not swim.

A Pocket Guide to Epidemiology 143 Subjects were asked to recall one week later whether they had swum for at least a half an hour on the day they were interviewed on the beach and whether they had developed a cold, cough or flu during the subsequent week. Since both exposure and illness information were obtained by subjects ' recall, it is reasonable to expect some subjects may incorrectly report either swimming or illness status or both. Suppose of the 367 subjects who got ill and swam, only 264 reported that they got ill and swam, and 30 subjects reported that they got ill but didn 't swim, 66 subjects reported that they did not get ill but swam, and 7 subjects reported that they did not get ill and did not swim. Suppose, further, that of the 233 subjects who truly got ill but did not swim, 130 reported this correctly, but 56 reported that they got ill and swam, 14 reported that they did not get ill but swam, and 33 reported that they did not swim and did not get ill. Continuing in this way, the table can be further revised to describe how the 300 subjects who truly did not get ill and swam were misclassified. The table can also be revised to describe the misclassification of the 1100 subjects who truly did not get ill and did not swim. We can now separately sum up the 4 frequencies within each of the 4 cells in the table of observed data to obtain a summarized table of the observed data as shown here: Study Questions (Q9.3) 1. What is the estimated risk ratio for the observed data? 2. Why is there misclassification bias? (Hint: RR=3.14 for true data) 3. What is the direction of the bias? 4. If there is misclassification of both exposure and disease, will the bias always be towards the null? Summary Misclassification can sometimes occur for both exposure and disease in the same study. An example of such misclassification is likely if both the exposure variable and the disease variable are determined by subject recall. When there is misclassification of both exposure and disease, the observed data results from how the cell frequencies in each of the four cells of the 2x2 table for the true data get split up into the four cells of the 2x2 table for the observed data.

144 Chapter 9. Are The Data Correct? Information Bias

Misclassification Probabilities – Sensitivity and Specificity
This misclassification table describes how a disease D may be misdiagnosed. Twelve subjects who were truly diseased were misclassified as non-diseased and 14 subjects who were truly not diseased were misclassified as diseased. 48 subjects who were truly diseased and 126 subjects who were truly non-diseased were correctly classified. In a perfect world, we would hope that no one was misclassified, that is, we would want our table to look like the table below. Then the proportion correctly classified as diseased would be 1 and the proportion correctly classified as non-diseased is also 1. In the real world, however, these proportions are not equal to one. In our example, the proportion of truly diseased correctly classified as diseased is .80 (48/60). The proportion of truly non-diseased correctly classified as non-diseased is .90 (126/140). The first of these proportions is called the sensitivity. Generally, the sensitivity for misclassification of disease status is the probability that a subject is classified as diseased given that he or she is truly diseased. The second of these proportions is called the specificity. Generally the specificity for misclassification of disease is the probability that a subject is classified as not diseased given that he or she is truly not diseased. The ideal value for both sensitivity and specificity is l.0 or 100%. We can also make use the misclassification table for exposure status to define sensitivity and specificity parameters.

A Pocket Guide to Epidemiology 145 Study Questions (Q9.4) Consider the numbers in the following misclassification table for exposure.

1. What is the sensitivity for misclassifying exposure? 2. What is the specificity for misclassifying exposure? 3. Do your answers to the previous questions suggest that there should be some concern about misclassification bias? Summary The underlying parameters that must be considered when assessing information bias are called sensitivity and specificity. Sensitivity gives the probability that a subject who is truly diseased (or exposed) will be classified as diseased (or exposed) in one’s study. Specificity gives the probability that a subject who is truly non-diseased (or unexposed) is classified as non-diseased (or unexposed) in one’s study. The ideal value for both sensitivity and specificity is 1, or 100%, which means there is no misclassification.

Nondifferential Misclassification
This table describes the true exposure and disease status for 2000 subjects in a hypothetical casecontrol study of the relationship between diet and coronary heart disease (CHD): The exposure variable is the amount of fruits and vegetables eaten in an average week, categorized as low or high, as recalled by the study subjects. The disease variable is the presence or absence of CHD. Suppose there is no misclassification of disease status, but that most subjects over-report their intake of fruits and vegetables because they think that diets with high amounts of fruits and vegetables are more acceptable to the investigator. In other words, there is misclassification of exposure status. The two tables that follow describe how exposure is misclassified separately for both the CHD cases and the non-cases.

146 Chapter 9. Are The Data Correct? Information Bias

Study Questions (Q9.5) 1. What are the sensitivity and specificity for the CHD cases? 2. What are the sensitivity and specificity for the non-cases? 3. What do these two misclassification tables have in common? This example illustrates non-differential misclassification of exposure. This occurs whenever the sensitivities and specificities do not vary with disease status. We have assumed that CHD status is not misclassified in this example. The sensitivities and specificities for misclassifying disease are all equal to 1 regardless of exposure group. Thus, in this example there is no misclassification of disease. Study Questions (Q9.5) continued 4. Use the column total in both misclassification tables (i.e., 600, 400, 300, and 700) to determine the odds ratio for the correctly (i.e., true) classified data. The row totals from each of the misclassification tables for exposure allow us to determine the study data that would actually be observed as a result of misclassification. Study Questions (Q9.5) continued 5. Why is there bias due to misclassifying exposure (Note: the correct odds ratio is 3.5)? 6. What is the direction of the bias? This example illustrates a general rule about nondifferential misclassification. Whenever there is nondifferential misclassification of both exposure and disease, the bias is always towards the null, provided that there are no other variables being controlled that might also be misclassified. Summary Nondifferential misclassification of disease: the sensitivities and specificities for misclassifying disease do not differ by exposure.

A Pocket Guide to Epidemiology 147 Nondifferential misclassification of exposure: the sensitivities and specificities for misclassifying exposure do not differ by disease. Nondifferential misclassification of both disease and exposure leads to a bias towards the null.
What Happens if a Variable other than Exposure or Disease Gets Misclassified? Greenland (1980) showed that if there is non-differential misclassification of exposure and disease, but also misclassification of a covariate, then there is no guarantee that there will be a bias towards the null. However, if misclassification of exposure and disease is nondifferential and a covariate that is not misclassified is controlled in the analysis (say, by stratification), then both stratum-specific and summary measures that adjust for the covariate will be biased towards the null. Other issues about misclassification of covariates were also addressed as follows: Misclassification of exposure can spuriously introduce effect modification (described in Chapter 10) by a covariate. Misclassification of a confounder (also described in Chapter 10) can reintroduce confounding in a summary estimate that controls for confounding using misclassified data.

Differential Misclassification
This table describes the true exposure and disease status for the same 2000 subjects described in the previous section for a hypothetical case-control study of the relationship between diet and coronary heart disease: Suppose, as before, there is no misclassification of disease status, but that subjects over-report their intake of fruits and vegetables because they think that diets with high amounts of fruits and vegetables are more acceptable to the investigator. Suppose also that a CHD case, who is concerned about the reasons for his or her illness, is not as likely to overestimate his or her intake of fruits and vegetables as is a control. Here are the two tables that describe how exposure is misclassified for both the CHD cases and controls:

Study Questions (Q9.6) The following questions are based on the previous two tables. 1. What are the sensitivity and specificity for the CHD cases? 2. What are the sensitivity and specificity for the non-cases? 3. Is there non-differential misclassification of exposure?

148 Chapter 9. Are The Data Correct? Information Bias This example illustrates differential misclassification of exposure. This occurs because the sensitivities and specificities for misclassifying exposure vary with disease status. The row totals from each of the misclassification tables for exposure allow us to determine the study data that would actually be observed as a result of misclassification: Study Questions (Q9.6) continued The following questions refer to the previous table and the table with the true exposure information shown previously. 4. Is there a bias due to misclassifying exposure? (Note, the correct OR is 3.5.) 5. What is the direction of the bias, if any? In general, when there is differential misclassification of either exposure or disease, the bias can be either towards the null or away from the null (see below for an example of bias away from the null).

Summary With differential misclassification, either the sensitivities and specificities for misclassifying D differ by E or the sensitivities and specificities for misclassifying E differ by D. Differential misclassification of either D or E can lead to bias either towards the null or away from the null.

Quantitative Assessment of Misclassification Bias
Let 's assume that we know there is a likely misclassification of exposure or disease that could bias our study results. How can we quantitatively correct for such bias to obtain an adjusted effect measure that is no longer biased? To quantify bias, we need to adjust the cell frequencies for the observed data to obtain a two-

A Pocket Guide to Epidemiology 149 way table of corrected cell frequencies from which we can compute a corrected effect measure. We can then compare the observed and possibly biased estimate with our corrected estimate to determine the extent of the possible bias and its direction. Study Questions (Q9.7) Use the information in the table below about the observed and corrected effect measures to determine whether the observed bias is towards or away from the null. Question Number a. b. c. d. e. f. g. h. Observed Effect 2.2 2.5 4.0 4.1 0.5 0.8 0.3 0.7 Corrected Effect 1.7 3.8 6.1 1.2 0.9 0.9 0.2 0.1 Towards the null? Away from the null?

Suppose we have determined that whatever bias that exists results from nondifferential misclassification of exposure or disease. 1. Do we need to obtain an adjusted effect measure that corrects for such bias? 2. How can we determine whether or not misclassification is nondifferential? Suppose we have determined that there is differential misclassification of exposure or disease. 3. Do we need to obtain an adjusted effect measure that corrects for possible misclassification bias? In presentations on Lesson Pages 9-3 and 9-4 of the ActivEpi CD, we give formulae for obtaining corrected estimates that consider non-differential or differential misclassification of either exposure or disease variables. Because these formulae are complex computationally, we do not provide them in the text below; the interested reader may work through the activities on 9-3 and 9-4 in the ActivEpi CD for further details. In any case, each of these formulae require reliable estimates of sensitivity and specificity. How can we determine the sensitivity and specificity if all we have are the observed data? One option is to take a small sample of the observed data for which true disease and exposure status is determined, so that sensitivities

150 Chapter 9. Are The Data Correct? Information Bias and specificities can be estimated (a drawback to this approach is the sample might be too small to give reliable values for sensitivity and specificity). Another option is to determine sensitivity and specificity parameters from separate data obtained in a previous study involving the same variables. A third option is simply to make an educated guess of the sensitivity and specificity parameters from your clinical or other knowledge about the study variables. A fourth option is to carry out what is often referred to as a sensitivity analysis with several educated guesses to determine a range of possible biases. Study Questions (Q9.7) continued Consider the following results of a sensitivity analysis for correcting for misclassification that is assumed to be nondifferential: Observed OR 1.5 1.5 1.5 1.5 Sensitivity? 80% 80% 90% 90% Specificity? 80% 90% 80% 90% Corrected OR 3.5 2.5 2.5 1.8

4. Why do the observed results compared to the corresponding corrected results illustrate a bias that is towards the null? 5. Which values of sensitivity and specificity are associated with the most bias? 6. Which values of sensitivity and specificity are associated with the least bias? 7. Based on the sensitivity analysis described above, how might you decide which corrected OR is “best”? Summary We can correct for misclassification bias by computing an adjusted effect measure from a two-way table whose cell frequencies are corrected from the misclassification found in observed cell frequencies. The correction requires accurate estimation of sensitivity and specificity parameters. Options for estimating sensitivity and specificity: o A sub-sample of the study data o A separate sample from another study o A questimate based on clinical or other theory/experience o A sensitivity analysis that considers several guestimates

Diagnostic Testing and Its Relationship to Misclassification
Diagnostic Test Studies
In clinical medicine, studies concerned with misclassification of disease are usually called diagnostic test studies. The primary goal of such a study is to evaluate the performance of a test for diagnosing a disease condition of interest. Suppose, for example, that the disease condition is deep vein thrombosis, or DVT. In a diagnostic study for DVT, the clinician targets only patients with a specific symptom, for example "acute leg swelling" and then performs both the diagnostic

A Pocket Guide to Epidemiology 151 test, typically an ultrasound, and the gold standard procedure, typically an x-ray venogram, on these patients. Here are the results of such a diagnostic test study in the form of a misclassification table. Using the diagnostic test, the disease classification status that is determined for a given patient is called the test result, and is labeled as positive “+” or negative “-“ on the rows of the table. The procedure used to define true disease is called the gold standard, however imperfect it may be. In the misclassification table, the results from using the gold standard are labeled on the columns of the table. Typically, the gold standard is a test that is more detailed, expensive, or risky than the diagnostic test used by the physician. The gold standard might even require prolonged follow-up of the patient if the disease is expected to eventually declare itself, post-mortem examination, or a measure combining more than one strategy, sometimes in complex ways tailored to the specific disease. Using the information in the misclassification table, the performance of a diagnostic test can be evaluated using several important measures, including the sensitivity, the specificity, and the prevalence. Recall that sensitivity describes the test 's performance in patients who truly have the disease, and is defined as the conditional probability of a positive test result given true disease, i.e.: P(Test + | True +) Study Questions (Q9.8) 1. What is the sensitivity of the test in the above table? 2. If the sensitivity had been 0.99, what could you conclude about a truly diseased patient who had a negative test result? Specificity describes the tests performance among patients who are truly without the disease. It is defined as the conditional probability of a negative test result given the absence of disease, P(Test - | True -). Study Questions (Q9.8) continued 3. What is the specificity of the test? 4. If the specificity had been .99, what would you conclude about a truly nondiseased patient who had a positive test result? Prevalence is calculated as the proportion of patients in the study sample who truly have the disease, P(True +). If little is known about a patient, disease prevalence in a diagnostic test study is the best estimate of pre-test probability that the patient has the disease. Study Questions (Q9.8) continued 5. What is the prevalence of true disease from these data? 6. Based on the sensitivity, specificity, and prevalence calculations above, do you think that the test is a good diagnostic tool for DVT? Explain briefly. Although the three measures, sensitivity, specificity, and prevalence provide

152 Chapter 9. Are The Data Correct? Information Bias important summary information about the performance of the diagnostic test, a more useful measure of overall performance is called the predictive value. It is described in the next section. Summary In clinical medicine, studies concerned with misclassification of disease are usually called diagnostic test studies. The purpose of a diagnostic test study is to evaluate test performance rather than to adjust for information bias. The procedure used to define true disease is called the gold standard. In a diagnostic study, the clinician targets patients with a specific symptom and then performs both the diagnostic test and the gold standard procedure on these patients. The performance of a diagnostic test can be evaluated using several important measures, including sensitivity, specificity, and prevalence. A more useful measure of the performance of a diagnostic test is provided by the predictive value.
Screening Tests A second type of clinical study concerned with misclassification is called a screening test. In contrast to a diagnostic test, a screening test targets a broad population of asymptomatic subjects to identify those subjects that may require more detailed diagnostic evaluation. The subjects in a screening test have not gone to a physician for a specific complaint. Members of the general public are typically invited to undergo screening tests of various sorts to separate them into those with higher and lower probabilities of disease. Those with higher probabilities are then urged to seek medical attention for definitive diagnosis. Those with lower probabilities receive no direct health benefit because they do not have the disease condition being screened. Also, depending on invasiveness of the screening test and/or the disease condition being targeted, persons under going screening may suffer risks as well as face some inconvenience, anxiety, personal cost, and sometimes discomfort, e.g., as with the use of a colonoscopy to screen for bowel cancer.

The Predictive Value of a Diagnostic Test
The probability of true disease status for an individual patient given the result of a diagnostic test is called the test 's predictive value. The predictive value is particularly useful to the clinician for individual patient diagnosis because it directly estimates the probability that the patient truly does or does not have the disease depending on the results of the diagnostic test. That 's what the clinician wants to know. Study Questions (Q9.9) Suppose T+ denotes the event that a patient truly has a disease condition of interest, whereas D+ denotes the event of a positive diagnostic test result on the same patient. 1. Which of the following two probability statements describes sensitivity and which describes predictive value? A. P(T+|D+) B. P(D+|T+)

A Pocket Guide to Epidemiology 153 The predictive value can be obtained directly from the misclassification table generated by a diagnostic test study. Because there are two possible results for a test, there are two different predictive values. The probability of actually having the disease when the test is positive is called the positive predictive value, and is denoted as PV+. The probability of actually not having the disease if the test is negative is the negative predictive value, and is denoted as PV-. Both PV+ and PV- are proportions. The closer these proportions are to 1, the better the test 's predictive performance. The positive predictive value is calculated as the number of true positive results divided by all positive results. Study Questions (Q9.9) continued 2. What is PV+ for the above table? The positive predictive value is often referred to as the post-test probability of having disease. It contrasts with prevalence, which gives the average patient 's pre-test probability of having disease. Study Questions (Q9.9) continued 3. Based on the data in the misclassification table, what is the estimate of the average patient’s probability of having DVT prior to performing an ultrasound? 4. Has the use of an ultrasound improved disease diagnosis for persons with positive ultrasound results? Explain briefly. The negative predictive value is the number of true negative results divided by the total number of subjects with negative test results. Study Questions (Q9.9) continued 5. What is PV- for these data? 6. Based on the data in the misclassification table, what is the estimate of the average patient’s probability of not having DVT prior to performing an ultrasound? 7. Has the use of an ultrasound improved disease diagnosis for persons with negative ultrasound results? Explain briefly. The prevalence of true disease in a diagnostic test study can greatly influence the size of the predictive values obtained. To illustrate this, we now consider a second misclassification table for a different group of patients who have presented to their clinician with pain but without swelling in their leg.

154 Chapter 9. Are The Data Correct? Information Bias

Study Questions (Q9.9) continued 8. What are the sensitivity, specificity, and prevalence in the table? 9. In the previously considered misclassification table, the sensitivity, specificity, and prevalence were 0.80, 0.90, and 0.30, respectively. How do these values compare with the corresponding values computed in the previous question? 10. What are the values of PV+ and PV- in the above table? 11. In the previously considered misclassification table, PV+ and PV- were .77 and .91, respectively, and the prevalence was .30. How do these values compare with the corresponding predicted values computed in the previous question? 12. What is the moral of this story relating predictive value to disease prevalence? Summary The predictive value (or post-test probability) is the probability of true disease status given the result of a diagnostic test. The predictive value can be obtained directly from the misclassification table generated by a diagnostic test study. The probability of disease when the test is positive is called the positive predictive value, and is denoted as PV+ The probability of disease when the test is negative is called the negative predictive value, and is denoted as PV-. PV+ = # true positives / all positive PV- = # true negatives / all negatives The closer PV+ and PV- are to 1, the better the test. The prevalence of true disease in a diagnostic test study can greatly influence the size of the predictive value. Quiz (Q9.10) For the classification table shown on the right, determine each of the following: 1. 2. 3. 4. What is the sensitivity? What is the specificity? What is the prevalence of the disease? What is the positive predictive value? ??? ??? ??? ???

Choices: 10% 33.3% 36.7%

81.8% 90%

A Pocket Guide to Epidemiology 155 The sensitivity and specificity for the classification table shown on the right are still 90% as in the previous questions. For this table, answer each of the following: 5. 6. 7. What is the prevalence of the disease? ??? What is the positive predictive value? ??? The prevalence in this table is smaller than in the previous table; therefore, the positive predictive value is ??? than in the previous table. 16.7% 64.3% 90% larger smaller

Choices:

Once again, the sensitivity and specificity for the classification table shown on the right are 90%. For this table, answer each of the following: 8. What is the prevalence of the disease? ??? 9. What is the positive predictive value? ??? 10. The prevalence in this table is smaller than in the previous two tables, therefore, the positive predictive value is ??? than in the previous two tables. 11. These results illustrate the fact that if the prevalence is small, the predictive value can be quite ??? even if the sensitivity and specificity parameters are quite ??? Choices: 10% 36.7% 50% high larger small smaller

Nomenclature
Misclassification tables for disease and exposure Misclassification table for disease: Truth D Not D D Classified Not D Misclassification table for exposure: Truth E Classified E Not E Not E

Observed (misclassified) and corrected tables Observed (i.e., misclassified) Data E Not E D a b Not D c d Corrected (i.e., adjusted) Data D Not D E A C Not E B D

156 Chapter 9. Are The Data Correct? Information Bias D D E E Not D Not D Not E Not E ˆ OR
ˆ OR adj

Truly has disease Classified as having disease Truly exposed Classified as exposed Truly does not have disease Classified as not having disease Truly not exposed Classified as not exposed Odds ratio from observed data = ad/bc Odds ratio from corrected or adjusted data = AD/BC

ˆ RR
ˆ RR adj

Risk ratio from observed data = [a/(a+c)]/[b/(b+d)] Risk ratio from corrected or adjusted data = [A/(A+C)]/[B/(B+D)] Of those truly with the characteristic, proportion correctly classified as having characteristic; for disease, Pr(D |D); for exposure, Pr(E |E) Sensitivity of disease misclassification Sensitivity of exposure misclassification Specificity of disease misclassification Specificity of exposure misclassification Of those truly without the characteristic, proportion correctly classified as not having the characteristic; for disease, P(not D |not D); for exposure, P(not E |not E).

Sensitivity SeD SeE SpD SpE Specificity

References on Information/Misclassification Bias
Overviews Greenberg RS, Daniels SR, Flanders WD, Eley JW, Boring JR. Medical Epidemiology (3rd Ed). Lange Medical Books, New York, 2001. Hill H, Kleinbaum DG. Bias in Observational Studies. In Encyclopedia of Biostatistics, pp 323-329, Oxford University Press, 1999. Kleinbaum DG, Kupper LL, Morgenstern H. Epidemiologic Research: Principles and Quantitative Methods. John Wiley and Sons Publishers, New York, 1982. Special Issues Barron, BA. The effects of misclassification on the estimation of relative risk. Biometrics 1977;33(2):414-8. Copeland KT, Checkoway H, McMichael AJ and Holbrook RH. Bias due to misclassification in the estimation of relative risk. Am J Epidemiol 1977;105(5):48895. Dosemeci M, Wacholder S, and Lubin JH. Does nondifferential misclassification of exposure always bias a true effect toward the null value? Am J Epidemiol 1990:132(4):746-8. Espeland MA, Hui SL. A general approach to analyzing epidemiologic data that contain misclassification errors. Biometrics 1987;43(4):1001-12. Greenland S. The effect of misclassification in the presence of covariates. Am J Epidemiol 1980;112(4):554-69. Greenland S, Kleinbaum DG. Correcting for misclassification in two-way tables and matched-pair studies. Int J Epidemiol 1983;12(1):93-7. Reade-Christopher SJ, Kupper LL. Effects of exposure misclassification on regression analyses of epidemiologic follow-up study data. Biometrics 1991;47(2):535-48. Satten GA and Kupper LL. Inferences about exposure-disease associations using probability-of-exposure information. J Am Stat Assoc 1993;88:200-8.

A Pocket Guide to Epidemiology 157
Wynder EL. Investigator bias and interviewer bias: the problem of reporting systematic error in epidemiology. J Clin Epidemiol 1994;47(8):825-7.

Answers to Study Questions and Quizzes
Q9.1 1. 2 2. 7 3. 100 x (6/8) = 75% 4. 100 x (7/10) = 79% Q9.2 1. False. Typically, several times and locations are used in the same residence, and a time-weighted average (TWA) is often calculated. 2. False. Good instrumentation for measuring time-weighted average has been available for some time. 3. False. A system of wire codes to measure distance and configuration has been used consistently since 1979 to rank homes crudely according to EMF intensity. However, the usefulness of this system for predicting past exposure remains an open question. 4. True 5. True 6. True (Note: there are no questions 7 to 9) 10. Interviewer bias. Subjects known to have experienced a venous thrombosis might be probed more extensively than controls for a history of oral contraceptive use. 11. Away from the null. The proportion of exposed among controls would be less than it should have been if both cases and controls were probed to the same extent. Consequently, the odds ratio in the misclassified data would be higher than it should be. 12. Keep the interviewers blind to casecontrol status of the study subject. 13. 5 14. 70 15. 100 x (95/100) = 95% 16. 100 x (70/80) = 87.5% Q9.3 1. The estimated risk ratio for the observed data is (380/1000)/(240/1000) = 1.58. 2. Because the observed risk ratio of 1.58 is meaningfully different than the true (i.e., correct) risk ratio of 3.14. 3. Towards the null, since the biased estimate of 1.58 is closer to the null value than is the correct estimate. 4. No way to tell from one example, but the answer is no, the bias might be either towards the null or away from the null. 5. The observed OR of 2.6 that results from misclassifying exposure is meaningfully different than the true odds ratio of 3.5. 6. The bias is towards the null. The biased OR estimated of 2.6 is closer to the null value of 1 than is the correct OR. Q9.4 1. Sensitivity = 720 / 900 = .8 or 80% 2. Specificity = 910 / 1100 = .83 or 83% 3. Yes. Both sensitivity and specificity are smaller than one. However, without correcting for the bias, it is not clear that the amount of bias will be large. Q9.5 1. Sensitivity = 480/600 = .80 or 80% and Specificity = 380/400 = .95 or 95%. 2. Sensitivity = 240/300 = .80 or 80% and Specificity = 665/700 = .95 or 95%. 3. The sensitivities for CHD cases and non-cases are equal. Also, the specificities for CHD cases and noncases are equal. The sensitivity information indicates that 20% of both cases and non-cases with low intake of fruits and vegetables tend to overestimate their intake. The specificity information indicates that only 5% of both cases and non-cases with high intake tend to under-estimate their intake. 4. In the correctly classified 2x2 table, a=600, b=400, c=300, and d=700, so the estimated odds ratio is ad/bc = (600 x 700) / (400 x 300) = 3.5. 5. The observed OR of 2.6 that results from misclassifying exposure is meaningfully different than the true odds ratio of 3.5. 6. The bias is towards the null. The biased OR of 2.6 is closer to the null value of 1 than the correct OR.

158 Chapter 9. Are The Data Correct? Information Bias
Q9.6 1. Sensitivity = 580/600 = .97 or 97% and Specificity = 380/400 = .95 or 95%. 2. Sensitivity = 240/300 = .80 or 80% and Specificity = 665/700 = .95 or 95%. 3. No. Although the specificities for cases and non-cases are equal (i.e., 95%), the sensitivity for the cases (97%) is quite different from the sensitivity for the non-cases (80%). This difference in sensitivities indicates that cases with low intake of fruits and vegetables are less likely to overestimate their intake than non-cases. 4. Not much. The observed OR of 3.95 that results from misclassifying exposure is slightly higher than the true odds ratio of 3.50. 5. The bias is slightly away from the null. The biased OR of 3.95 is further away from the null value of 1 than is the correct OR of 3.5. Q9.7 a) Away b) Towards c) Towards d) Away e) Away f) Away g) Towards h) Towards 1. It depends. We know that the bias must be towards the null. If the direction of the bias is all that we are interested in, then we do not need to correct for the bias. However, if we want to determine the extent of the bias and to obtain a quantitative measure of the true effect, then we need to correct for the bias. 2. We can either reason that misclassification is nondifferential from our knowledge or experience with the exposure and disease variables of our study, or we can base our decision on reliable estimates of the sensitivity and specificity parameters. 3. It depends. The bias may be either towards the null or away from the null. We might be able to determine the direction of the bias by logical reasoning about study characteristics. Otherwise, the only way we can determine either the extent or direction of the bias is to compare a corrected estimate with an observed estimate. 4. The biased (i.e., misclassified) observed odds ratio is closer to the null than the corrected odds ratio. 5. The greatest amount of bias is seen with the observed OR is 1.5 compared to the corrected OR of 3.5, which occurs when both the sensitivity and specificity are 80%. 6. The bias is smallest when the correct OR is 1.8, which results when both sensitivity and specificity are 90%. 7. One way to decide is to choose the corrected OR corresponding to the most realistic set of values for sensitivity and specificity. Another way is to choose the corrected OR (here, 3.5) that is most distant from the observed OR. A third alternative is to choose the corrected OR that changes least (here, 1.8) from the observed OR. Q9.8 1. Sensitivity = 48 / 60 = 0.80. 2. The patient is very unlikely to have the disease, since the probability of getting a negative test result for a patient with the disease is.01, which is very small. 3. Specificity = 126 / 140 = 0.90 4. The patient is very likely to have the disease, because the probability of getting a positive result for a patient without the disease is .01, which is very small. 5. Prevalence of true disease = 60 / 200 = 0.30. 6. Cannot fully answer this question. Both the sensitivity and specificity are relatively high at .80 and .90, but the prevalence is only 30%. What is required is the proportion of total ultrasound positives that truly have DVT, which in this study is 48 / 62 = 0.77, which is high but not over .90 or .95. Q9.9 1. Choice A is the predictive value and Choice B is sensitivity. 2. PV+ = 48 / 62 = 0.77 3. Based on the table, the prior probability of developing DVT is 60 / 200 = 0.30,

A Pocket Guide to Epidemiology 159 which is the estimated prevalence of disease among patients studied. Yes, the prior probability was 0.30, whereas the (post-test) probability using an ultrasound increased to 0.77 given a positive result on the test. PV- = 126 / 138 = 0.91 Based on the table, the prior probability of not developing DVT is 140 / 200 = 0.70, which is 1 minus the estimated prevalence of disease among patients studied. Yes, the prior probability of not developing DVT was 0.70 whereas the (post-test) probability of not developing DVT using an ultrasound increased to 0.91 given a negative test result. Sensitivity = 16 / 20 = 0.80, specificity = 162 / 180 = 0.90, prevalence = 20 / 200 = .10. Corresponding sensitivity and specificity values are identical in both tables, but prevalence computed for this data is much lower at 0.10 than computed for the previous table (.30). PV+ = 16 / 34 = 0.47 and PV- = 162 / 166 = 0.98. 11. PV+ has decreased from 0.77 to 0.47 and PV- has increased from 0.91 to 0.98 whereas the prevalence has dropped from 0.30 to 0.10 while sensitivity and specificity has remained the same and high. 12. If the prevalence decreases, the predictive value positive will decrease and may be quite low even if sensitivity and specificity are high. Similarly, the predictive value negative will increase and may be very high, even if the sensitivity and specificity are not very high. Q9.10 1. 90% 2. 90% 3. 33.3% 4. 81.8% 5. 16.7% 6. 64.3% 7. smaller 8. 10% 9. 50% 10. smaller 11. small, high

4.

5. 6.

7.

8.

9.

10.

CHAPTER

10

OTHER FACTORS ACCOUNTED FOR? CONFOUNDING AND INTERACTION
Confounding is a form of bias that concerns how a measure of effect may change in value depending on whether variables other than the exposure variable are controlled in the analysis. Interaction/effect modification, which is different from confounding, compares estimated effects after other variables are controlled.

The Concept of Confounding
Confounding is an important problem for health and medical researchers whenever they conduct studies to assess a relationship between an exposure, E, and some health outcome or disease of interest, D. Confounding is a type of bias that may occur when we fail to take into account other variables, like age, gender, or smoking status, in attempting to assess an E D relationship. To illustrate confounding consider the results from a hypothetical retrospective cohort study to determine the effect of exposure to a suspected toxic chemical on the development of lung cancer for workers in a chemical industry. We will call the chemical TCX. The ten-year risks for lung cancer are estimated to be 0.36 for those who were exposed to TCX and 0.17 for those who were not exposed to TCX. The estimated risk ratio is 2.1, which indicates that those exposed to TCX have twice the risk for lung cancer as those unexposed. So far, we have considered two variables, exposure to TCX, and lung cancer status, the health outcome. We haven 't yet considered any other variables that might also have been measured or observed on the patients in this study. For example, we might wonder whether there were relatively more smokers among those who were exposed to TCX than those unexposed to TCX. If so, that may explain why workers exposed to TCX were found to have an increased risk of 2.1 compared to unexposed workers. Those exposed to TCX may simply have been heavier smokers and, therefore, more likely to develop lung cancer than among those not exposed to TCX, regardless of exposure to TCX. Perhaps TCX exposure is a determinant of some other form of cancer or another disease, but not necessarily lung cancer. Suppose that we categorize our study data into two smoking history categories, non-smokers and smokers. For these tables, the estimated risk ratio is computed to be 1.0 for non-smokers and 1.3 for smokers. Notice that these two stratum-specific risk ratios suggest no association between exposure to TCX and the development of lung cancer.

162 Chapter 10. Other Factors Accounted For? Confounding

When we form strata by categorizing the entire dataset according to one or more variables, like smoking history in our example here, we say that we are controlling for these variables, which we often refer to as control variables. Thus, what looks like a twofold increase in risk when we ignore smoking history, changes to no association when controlling for smoking history. This suggests that the reason why workers exposed to TCX had a twofold increase in risk compared to unexposed workers might be explained simply by noting that there were relatively more smokers among those exposed to TCX. This is an example of what we call confounding, and we say that smoking history is a confounder of the relationship between TCX exposure status and ten-year risk for lung-cancer. In general, confounding may be described as a distortion in a measure of association, like a risk ratio, that may arise because we fail to control for other variables, for example, smoking history, that might be risk factors for the health outcome being studied. If we fail to control the confounder we will obtain an incorrect, or biased, estimate of the measure of effect. Summary Confounding is a distortion in a measure of effect, e.g., RR, that may arise because we fail to control for other variables, for example, smoking history, that are previously known risk factors for the health outcome being studied. If we ignore the effect of a confounder, we will obtain an incorrect, or biased, estimate of the measure of effect. Quiz (Q10.1) 1. Confounding is a ??? in a ??? that may arise because we fail to ??? other variables that are previously known ??? for the health outcome being studied. Choices case-control study control for distortion effect modifiers eliminate measure of effect risk factors

A Pocket Guide to Epidemiology 163 A study finds that alcohol consumption is associated with lung cancer, crude OR = 3.5. Using the data below, determine whether smoking could be confounding this relationship. 2. What is the OR among smokers? 3. What is the OR among non-smokers? 4. Does the OR change when we control for smoking status? Choices 0.01 1.0 3.5 5.4 ??? ??? ???

5. Is there evidence from data shown below that smoking is a confounder of the relationship between alcohol consumption and lung cancer? ??? Choices no yes

Crude versus Adjusted Estimates
Confounding is assessed in epidemiologic studies by comparing the crude ˆ estimate of effect (e.g., cRR ) in which no variables are controlled, with an ˆ adjusted estimate of effect (e.g., aRR ), in which one or more variables is controlled. The adjusted estimate is typically computed by combining stratumspecific estimates into a single number. For example, to assess confounding by smoking history in the previously described retrospective cohort study of the effects of exposure to the chemical TCX on the development of lung cancer, we can compare the crude risk ratio of 2.1 to an adjusted risk ratio that combines the risk ratios of 1.0 and 1.3 for the two smoking history categories. The method for combining these estimates into a single summary measure involves computing a weighted average of stratumspecific measures of effect; see Chapter 14 on Stratified Analysis for more details. Once we have combined these stratum specific estimates, how do we decide if there is confounding? The data-based criterion for confounding requires the crude estimate of effect to be different from the adjusted estimate of effect. How different must these two estimates be to conclude that there is confounding? To answer this question, the investigator must decide whether or not there is a clinically important difference. In our retrospective cohort study the adjusted estimate would be some number between 1.0 and 1.3, which suggests a much weaker relationship than indicated by the crude estimate of 2.1. Most investigators would consider this a clinically important difference. Suppose the crude estimate had been 4.2 instead of 2.1, the difference between crude and adjusted estimates would indicate even much stronger confounding. We can compare other crude and adjusted estimates. Suppose, for example that

164 Chapter 10. Other Factors Accounted For? Confounding an estimated crude risk ratio was 4.2 and the estimated adjusted risk ratio was 3.8. Both these values indicate an association that is about equally strong, so there is no clinically important difference between crude and adjusted estimates. Similarly, if the estimated crude risk ratio is 1.2 and the estimated adjusted risk ratio is 1.3, both these values indicate about the same very weak or no association. So, here again, there is no clinically important difference between these crude and adjusted estimates.

Risk Ratio (RR)

Clearly, deciding on what is clinically important requires a subjective decision by the investigators. One investigator might conclude, for example, that the difference between a 1.2 and a 1.3 is clinically important whereas another investigator might conclude otherwise. This problem may lead one to want to use a test of statistical significance to decide on whether there is a difference between the crude and adjusted estimate. However, because confounding is a validity issue, it should not be evaluated using a statistical test, but rather by looking for a meaningful difference, however imprecise. A commonly used approach for assessing confounding is to specify, prior to looking at one 's data, how much of a Risk Ratio (RR) change in going from the crude to the adjusted estimate is required. Typically, a 10 per cent change is specified, so that if the crude risk ratio estimate is say, 4, then a 10% change in this estimate either up or down would be obtained for an adjusted risk ratio of either 3.6 or 4.4. Thus, if the Risk Ratio (RR) adjusted risk ratio were found to be below 3.6 or above 4.4, we would say that confounding has occurred with at least a 10% change in the estimated association. As another example, if a 20% change is specified and the crude risk ratio is, say, 2.5, the adjusted risk ratio would have to be either below 2 or above 3 to conclude that there is confounding. Summary Confounding is assessed by comparing the crude estimate of effect, in which no variables are controlled, with an adjusted estimate of effect, in which one or more variables are controlled. Confounding is present if we conclude that there is a clinically important or meaningful difference between crude and adjusted estimates.

A Pocket Guide to Epidemiology 165 We do not use statistical testing to evaluate confounding. A commonly used approach for assessing confounding is to specify, prior to looking at one’s data, how much of a change in going from the crude to the adjusted estimate is required. Quiz (Q10.2) A case-control study was conducted to study the relationship between oral contraceptive use and ovarian cancer. The crude OR was calculated as 0.77. Since age was considered a potential confounder in this study, the data were stratified into 3 age groups as shown below. 1. 2. 3. 4. The OR for the 20-39 year age group is ??? The OR for the 40-49 year age group is ??? The OR for the 50-54 year age group is ??? Do you think that this data provides some evidence that age is a confounder of the relationship between oral contraceptive use and ovarian cancer? ??? 0.61 0.65 0.69 0.77 1.45 no yes

Choices: 0.58

In this study described in the previous question, the crude OR was 0.77, and the adjusted OR controlling for age was 0.64. 5. If a 15% change in the crude versus adjusted OR is specified by the investigator as a meaningful difference, an adjusted OR less than ??? or greater than ??? provides evidence of confounding. 6. Is there evidence of confounding? ??? 7. Suppose the investigators determined that a 20% change was a meaningful difference. Is there evidence of confounding? ??? Choices: 0.15 0.63 0.65 0.85 0.89 0.92 no yes

Quiz (Q10.3) Determine whether each of the following is True or False. 1. An adjusted estimate is a suitably chosen weighted average of the stratum specific estimates. ???. 2. An adjusted estimate is always less than the corresponding crude estimate. ??? 3. Most epidemiologists prefer to give equal weight to each stratum specific estimate in case-control studies. .???. 4. Confounding is a validity issue and therefore, requires the use of a statistical test to determine its significance. ???. Use the formula below to calculate the adjusted RR for the following examples whose stratum specific estimates are given. (Note: Although the formula below gives a weighted average, the usual formula for aRR is a more complicated “precision-based” weighted average described in Chapter 12 on Stratified Analysis.)

166 Chapter 10. Other Factors Accounted For? Confounding

5. Stratum 1: RR=1.13, w=13.1; Stratum 2: RR=1.00, w=7.7. The adjusted RR is ???. 6. Stratum 1: RR=2.25, w=31.3; Stratum 2: RR=1.75, w=5.6. The adjusted RR is ??? Choices: 1.06 1.07 1.08 1.09 1.98 2.08 2.17

Criteria for Confounding
In addition to the data-based criterion for confounding, we must assess several a priori criteria. These are conditions to consider at the study design stage, prior to data collection, to identify variables to be measured for possible control in the data analysis. The first a priori criterion is that a confounder must be a risk factor for the health outcome. This criterion ensures that a crude association between exposure and disease cannot be explained away by other variables already known to predict the disease. Such variables are called risk factors. For example, suppose we are studying the link between exposure to a toxic chemical and the development of lung cancer in a chemical industry. Based on the epidemiologic literature on the determinants of lung cancer, we would want to control for age and smoking status, two known risk factors. Our goal is to determine whether exposure to the chemical contributes anything over and above the effects of age and smoking on the development of lung cancer. The second criterion is that a confounder cannot be an intervening variable between the exposure and the disease. A pure intervening variable (V) is any variable whose relationship to exposure and disease lies entirely within the causal pathway between exposure and disease. Given a hypothetical scenario where saturated fat levels are measured to determine their effects on CHD, would we want to control for LDL levels? (Note: use of LDL in this example might The answer: If we control for LDL level, we essentially control for the saturated fat level, and we would likely find an adjusted risk ratio or odds ratio relating saturated fat to coronary heart disease status to be close to the null value. The intervening variable here is LDL level, and we should not control for it.

LDL

The third criterion is that a confounder must be associated with the exposure in the source population being studied. By source population we mean the underlying population cohort that gives rise to the cases used in the study. Consider a study to assess whether a particular genetic

A Pocket Guide to Epidemiology 167 factor, BRCAl, is a determinant of breast cancer. Age is a well-known risk factor for breast cancer, but is clearly not associated with the presence or absence of the gene in whatever source population is being studied. The third a priori criterion is therefore not satisfied. Consequently, even if by some chance, age turned out to be associated with the gene in the study data, we would not control for age, even though there is data-based confounding, because age does not satisfy all a priori criteria. All three a priori criteria plus the data-based criterion are required for a variable to be considered a true confounder.

Summary A confounder must satisfy 3 a priori criteria in addition to the data-based criterion for confounding. These are: A confounder must be a risk factor for the health outcome. A confounder cannot be an intervening variable in the causal pathway between exposure and disease. A confounder must be related to exposure in the source population from which the cases are derived.
Some "fine points" about risk factors The decision regarding which variables to include in the list of risk factors is, in practice, rarely a clear-cut matter. Such is the case when only a small amount of literature is available on a given study subject. On the other hand, a large literature may be controversial in terms of which previously studied variables are truly predictive of the disease. Also, after data collection, but prior to the primary data analysis, the list of risk factors may need to be re-evaluated to allow for the addition or deletion of variables already measured but not explicitly considered for control. Variables measured for other purposes, say in a broad study to evaluate several etiologic questions, may be added to the list of risk factors if they were previously overlooked. Furthermore, a surrogate of a risk factor may have to be used when the latter is difficult to measure. For example, the number of years spent in a given job in a particular industry is often used as a surrogate measure for the actual amount of exposure to a toxic substance suspected of being an occupationally related carcinogen. Some "fine points" about a priori criterion 3 The third a priori criterion for confounding is of particular concern in case-control studies, where the controls are usually selected into the study after the cases have already occurred. In such studies, it is possible that the study data are not representative of the source population with regard to the exposure as well as other variables. Therefore, a variable, say C, that may be not associated with exposure in the source population may still be associated with the exposure in the actual study data. In such a case, criterion 3 says that the variable C cannot be considered a confounder, and should not be

168 Chapter 10. Other Factors Accounted For? Confounding controlled, even if there is data-based confounding. In cohort studies, the exposure status is determined before disease status has occurred, so that the source population is the study cohort. In such studies, a variable, C, that is not associated with the exposure in the study data, does not satisfy condition 3 and therefore should not be considered a confounder. The main difficulty in assessing the third a priori criterion concerns how to determine the association of the suspected confounder, C, with the exposure, E, in the in the source population. This requires some knowledge of the epidemiologic literature about the relationship between C and E and about the source population being studied

Quiz (Q10.4) A study was conducted to assess the relationship between blood type (O-positive and O-negative) and a particular disease. Since age is often related to disease outcome, it was considered a potential confounder. Determine whether each of the a priori criteria below is satisfied. 1. A confounder must be a risk factor for the health outcome. ??? 2. A confounder cannot be an intervening variable between the exposure and the disease. ??? 3. A confounder must be associated with the exposure in the source population being studied. ??? 4. Can age be a confounder in this study? ??? Choices: no not satisfied satisfiedyes

Confounding in Different Study Designs
We have seen that the assessment of confounding requires both data-based and apriori criteria. The data-based criterion requires that the crude estimate of effect be meaningfully different from the adjusted estimate of effect. The adjusted estimate of effect is computed as a weighted average of stratum-specific estimates obtained over different categories of the potential confounder. The measure of effect used for this comparison changes with the study design but it always compares a crude estimate with an adjusted one. We have thus far only considered follow-up studies where the measure of effect of interest is the risk ratio. For case-control studies, we compare crude and adjusted estimates of the exposure odds ratio. In cross-sectional studies, we compare crude and adjusted estimates of the prevalence odds ratio or the prevalence ratio. Summary The measure of association used to assess confounding will depend on the study design. In a follow-up study, we typically compare a crude risk ratio with an adjusted risk ratio. In a case-control study, we typically compare a crude exposure odds ratio with an adjusted exposure odds ratio.

A Pocket Guide to Epidemiology 169 In a cross-sectional study, we typically compare a crude prevalence odds ratio with an adjusted prevalence odds ratio, or we might use a prevalence ratio instead of a prevalence odds ratio. Regardless of the study design, data based confounding is assessed by comparing a crude estimated of effect with an appropriate adjusted estimate of effect.

Assessing Confounding in Case-Control Studies
These tables show results from a case-control study to assess the relationship of alcohol consumption to oral cancer. The tables describe the crude data when age is ignored and the stratified data when age has been categorized into three groups. The investigators wanted to evaluate whether there was possible confounding due to age. From the above data, we can conclude the following: 1. The crude odds ratio relating alcohol to oral cancer is calculated by using the formula (a x d)/(b x c) which in this case equals 2.8 2. The stratum-specific odds ratio relating alcohol to oral cancer can be calculated for each strata by using the formula (a x d)/(b x c). The stratum-specific odds ratios are 2.2 for the 40 to 49 age group, 2.2 for the 50 to 59 age group and 2.0 for the 60 and higher age group. 3. There is data-based confounding because the crude odds ratio of 2.8 is meaningfully different than any weighted average of stratum-specific odds ratios, all of which are about 2. 4. There is data-based confounding because the crude odds ratio of 2.8 is meaningfully different than any weighted average of stratum-specific odds ratios, all of which are about 2. 5. Age is a confounder provided all three a priori conditions for confounding are assumed to be satisfied. Summary As an example to assess confounding involving the exposure odds ratio, we consider a case-control study of the relationship between alcohol consumption and oral cancer. Age, a possible confounder, has been categorized into three groups. The crude estimate of 2.8 indicates a threefold excess risk whereas the adjusted estimate of 2.1 indicates a twofold excess risk for drinkers over non-drinkers. The results indicate that there is confounding due to age.

170 Chapter 10. Other Factors Accounted For? Confounding Quiz (Q10.5) These data are from a cross-sectional seroprevalence survey of HIV among prostitutes in relation to IV drug use. The crude prevalence odds ratio is 3.59. 1. What is the estimated POR among the black or Hispanic group? ??? 2. What is the estimated POR among the whites? ??? 3. Which table do you think should receive more weight when computing an adjusted odds ratio? ??? Choices: 3.25 3.59 4.00 4.31 4.69 Black or Hispanic White

In the study described in the previous questions, the estimated POR for the Black or Hispanic group was 4.00 and the estimated POR for the Whites was 4.69. The “precision-based” adjusted POR for this study is 4.16. Recall that the crude POR was 3.59. 4. Is there confounding? . Choices maybe no yes . . .. ???

Confounding versus Interaction
Another reason to control variables in an epidemiologic study is to assess for interaction. To assess interaction, we need to determine whether the estimate of the effect measure differs at different levels of the control variable. Consider the results from a case-control study to assess the potential relationship between alcohol consumption and bladder cancer. These data are stratified on race in three categories.

2.21
The estimated odds ratios for the three race strata and the combined strata are computed to be 1.74, 1.72, 3.75, and 1.73. There is clear evidence of interaction here because the effect is strong in Asians, but less so in Whites and Blacks. It is not clear whether there is confounding, since the value of the adjusted estimate could vary between 1.72 and 3.75, depending on the weights assigned to the strata. The precision-based adjusted odds ratio is computed to be 2.16, which is not very different from the crude odds ratio of 1.73, suggesting that there is little evidence

A Pocket Guide to Epidemiology 171 of confounding. Confounding and interaction are different concepts. Confounding compares the estimated effects before and after control whereas interaction compares estimated effects after control. When assessing confounding and interaction in the same study, it is possible to find one with or without the other. Consider the table shown here giving stratum specific and crude risk ratio estimates from several hypothetical data sets in which one dichotomous variable is being considered for control. For each data set in this table, do you think there is interaction or confounding? Think about your answer for a few minutes, and then continue to see the answers below.

Let us look at the data sets, one at a time. For dataset 1, there is clearly interaction, because the estimate for stratum 1 indicates no association but the estimate for stratum 2 indicates a reasonably strong association. There is clearly confounding, because any weighted average of the values 1.02 and 3.50 will be meaningfully different from the crude estimate of 6.0. For data set 2, again there is clearly interaction, as in data set 1. However, it is not clear whether or not there is confounding. The value of an adjusted estimate will depend on the weights assigned to each stratum. If all the weight is given to either stratum 1 or stratum 2, then the crude estimate of 2.0 will differ considerably from the adjusted estimate, but if equal weight is given to each stratum, the adjusted estimate will be much closer to the crude estimate. Nevertheless, the use of an adjusted estimate here is not as important as the conclusion that the E D association is different for different strata. Dataset 3 also shows interaction, although this time the nature of the interaction is different from what we observed in datasets 1 and 2. Here, the two stratum specific estimates are on opposite sides of the null risk ratio value of 1. It appears there is a protective effect of exposure on disease in stratum 1, but a harmful effect of exposure on disease in stratum 2. In this situation, the assessment of confounding is questionable and potentially very misleading, since the important finding here is the interaction effect, especially if this strong interaction holds up after performing a statistical test for interaction. In dataset 4, the two stratum specific estimates are identically equal to one, so there is no interaction. However, there is clear evidence of confounding, since the crude estimate of 4.0 is meaningfully different from both stratum-specific estimates. In dataset 5, there is no interaction, because the stratum-specific estimates are

172 Chapter 10. Other Factors Accounted For? Confounding both equal to 4. There is also no confounding because the crude estimate is essentially equal to both stratum specific estimates.

Summary Confounding and interaction are different concepts. Interaction considers what happens after we control for another variable. Interaction is present if the estimate of the measure of association differs at different levels of a variable being controlled. When assessing confounding and interaction in the same study, it is possible to find one with or without the other. In the presence of strong interaction, the assessment of confounding may be irrelevant or misleading. Quiz (Q10.6) 1. In contrast to ??? when interaction is present, the estimates of the ??? differ at various levels of the control variable. 2. When assessing confounding and interaction in the same study, it is ??? to find one without the other. Choices confounding effect measure not possible possible precision effect modification variance

For datasets 1-3 in the table below, select the best answer from the following: A. Confounding B. Interaction C. No confounding or interaction D. Calculation error (not possible) 3. Data set 1 ? ??? 4. Data set 2? ??? 5. Data set 3? ???

A Pocket Guide to Epidemiology 173

Interaction versus Effect Modification
The term effect modification is often used interchangeably with the term interaction. We use effect modification from an epidemiologic point of view to emphasize that the effect of exposure on the health outcome is modified depending on the value of one or more control variables. Such control variables are called effect modifiers of the relationship between exposure and outcome. We use interaction from a statistical point of view to emphasize that the exposure variable and the control variable are interacting in some way within a mathematical model for determining the health outcome. To illustrate effect modification, consider the case-control data to assess the relationship between alcohol consumption and bladder cancer. The data showed clear evidence of interaction, since the estimated effect was much stronger in Asians than in either Blacks or Whites. This evidence suggests that race is an effect modifier of the relationship between alcohol consumption and bladder cancer. Such a conclusion is supported by the epidemiologic literature, which indicates that alcohol is metabolized in Asians differently than in other racial groupings.

2.21 The assessment of interaction or effect modification is typically supported using the results of statistical testing. Recall that confounding does not involve significance testing because it is a validity issue. Nevertheless, statistical testing of interaction is considered appropriate in epidemiologic studies because effect modification concerns understanding the underlying causal mechanisms involved in the E D relationship, which is not considered a validity issue. One such test for stratified data that has been incorporated into available computer software, is called the Breslow-Day test Summary Effect modification and interaction are often used interchangeably. If there is effect modification, then the control variable or variables involved are called effect modifiers. The assessment of effect modification is typically supported by statistical testing for significant interaction. One popular statistical test for interaction is called the Breslow-Day test. Is There Really a Difference between Effect Modification and Interaction?
Although the terms effect modification and interaction are often used interchangeably, there is some controversy in the epidemiologic literature about the precise definitions of effect modification and interaction (see Kleinbaum et al., Epidemiologic Research: Principles and Quantitative Methods, Chapter 19, John Wiley and Sons, 1982).

One distinction frequently made is that effect modification describes a non-

174 Chapter 10. Other Factors Accounted For? Confounding quantitative clinical or biological attribute of a population, whereas interaction is typically quantitative and data-specific, and in particular, depends on the scale on which the "interacting" variables are measured. Nevertheless, this conceptual distinction is often overlooked in the applied research studies. Why Do Epidemiologists Statistically Test for Interaction but not for Confounding?
We have previously pointed out that the assessment of confounding should not involve statistical testing, essentially because confounding is a validity issue involving systematic rather than random error. Moreover, if there is a meaningful difference between estimated crude and adjusted effects, then a decision has to be made as to which of these estimates to report; consequently, the adjusted effect must be used, without consideration of a statistical test, because it controls for the variables (i.e., risk factors) designated for adjustment. Furthermore, it is not obvious, even if we wanted to statistically test for confounding, exactly how to properly perform a test for confounding. What is typically done, though incorrect, is to test whether the potential confounder, e.g., age, is significantly associated with the health outcome, possibly also controlling for exposure status. Such a test does not really assess confounding, since it concerns random error (i.e., variability) rather than whether or not the crude and adjusted estimates are different in the data! As to whether or not one should do a statistical test for assessing interaction/effect modification, the answer is not as clear-cut. If we consider interaction as a data-based manifestation of a population-based phenomenon (i.e., effect modification), then a statistical test can be justified to account for the random error associated with a data-based result. Moreover, in contrast, to the assessment of confounding, there are several 'legitimate ' approaches to testing for interaction, one of which is the Breslow-Day test for stratified data (described in Chapter 14) and another is a test for the significance of product terms in a logistic model. Furthermore, it may be argued that the assessment of interaction/effect modification isn 't a validity issue, but rather concerns the conceptual understanding/explanation of the relationships among variables designated for control. The latter argument, in this author 's opinion, is a little too esoteric to accept at face value. In fact, a counter argument can be made that the presence of interaction/effect modification implies that the most "valid" estimates are obtained by stratifying on effect modifiers, provided that one can determine which variables are the "true" effect modifiers.

As in many issues like this one that arise in the undertaking of epidemiologic research, the best answer is probably, "it depends!" That is, it depends on the researcher 's point of view whether or not a statistical test for interaction/effect modification is appropriate. Nevertheless, this author tends to weigh in with the opinion that effect modification is a population phenomenon that can be assessed using 'legitimate ' statistical testing of a data-based measure of interaction.

Effect Modification – An Example
In the early 1990 's, investigators of the Rotterdam Study screened 8,000 elderly men and women for the presence of Alzheimer 's disease. One of the research questions was whether the presence of atherosclerosis increased the risk of this disease. In a cross-sectional study, the investigators found that patients with high levels of atherosclerosis had a three times increased risk of having Alzheimer 's disease compared to participants with only very little atherosclerosis. These results were suggestive of a link between cardiovascular disease and neurodegenerative disease. The investigators knew from previous research that one of the genes involved

A Pocket Guide to Epidemiology 175 in lipid metabolism influences the risk of Alzheimer 's disease. For this gene, there are two alternative forms, allele A and allele B. Each person 's genetic make-up consists of two of these alleles. Persons with at least one B-allele have a higher risk of Alzheimer 's disease than persons with two A-alleles. The investigators hypothesized that a person 's genetic make-up might modify the association they found between atherosclerosis and Alzheimer 's disease. Therefore, they divided the study population into a group of participants who had at least one B-allele, and a group of participants with two A-alleles. They found the following results: Among those with at least one B allele, the prevalence of Alzheimer 's disease for those with high levels of atherosclerosis was three times the prevalence of those with low levels. This result is the same as the crude. However, among those with two A-alleles, the prevalence for those with high levels of atherosclerosis was only 1.4 times the prevalence of those with low levels. These results provide an example of effect modification. Genetic make-up is the effect modifier. The investigators showed that the extent of atherosclerosis is associated with Alzheimer 's disease, but only in those whose genetic make-up predisposes them to the development of this disease. Study Questions (Q10.7)

The prevalence of Alzheimer’s and the prevalence ratios for each gene group are listed above. Answer the following assuming the group with low ATH and two Aalleles is the reference. 1. What is the prevalence ratio comparing those with high ATH and two Aalleles to the reference group? 2. What is the prevalence ratio comparing those with low ATH and at least one B-allele to the reference group? 3. What is the prevalence ratio comparing those with both high ATH and at least one B-allele to the reference group? 4. What is the difference between the three prevalence ratios you just calculated and the two listed above?

176 Chapter 10. Other Factors Accounted For? Confounding Quiz (Q10.8) True or False

1. The term effect modification emphasizes that the effect of exposure on the health outcome is modified depending on the value of one or more control variables. ??? 2. Evidence for effect modification is present when the stratum-specific measures of association are approximately the same. ??? 3. This assessment can be supported by a statistical test known as the BreslowDay test. ??? A measles vaccine may be highly effective in preventing disease if given after a child is 15 months of age, but less effective if given before 15 months. 4. This example illustrates ???, where the exposure is ???, the outcome is ???, and the effect modifier is ???. Choices: age at vaccination measles measles vaccine confounding effect modification

Nomenclature
ˆ aEOR ˆR aPO

ˆ aPR
ˆ aRR

C
ˆ cEOR

ˆ cPOR ˆ cPR
ˆ cRR

D E V w or wi

Estimate of the adjusted exposure odds ratio Estimate of the adjusted prevalence odds ratio Estimate of the prevalence ratio Estimate of an adjusted risk ratio Confounding variable Estimate of the crude exposure odds ratio Estimate of the prevalence odds ratio Estimate of the prevalence ratio Estimate of the crude risk ratio Disease Exposure Intervening variable Weight; with a subscript i, denotes the weight for a stratum

References
Hofman A, Ott A, Breteler MM, Bots ML, Slooter AJ, van Harksamp F, van Duijn CN, Van Broeckhoven C, Grobbee DE. Atherosclerosis, apolipoprotein E, and prevalence of dementia and Alzheimer 's disease in the Rotterdam Study. Lancet 1997;349 (9046):151-4. Greenland S, Morgenstern H. Confounding in health research. Annu Rev Public Health 2001;22:189-212. Kleinbaum DG, Kupper LL, Morgenstern H. Epidemiologic Research: Principles and Quantitative Methods. John Wiley and Sons Publishers, New York, 1982. Miettinen O. Confounding and effect modification. Am J Epidemiol 1974;100(5):350-3. Mundt KA, Shy CM. Interaction: An epidemiological perspective for risk assessment. In: Fan AM, Chang LW (eds.). Toxicology and risk assessment: Principles, methods, and applications. Marcel Dekker, Inc., New York, NY, p.p. 329-351, 1996

A Pocket Guide to Epidemiology 177
Whittemore AS. Collapsibility of multidimensional contingency tables. J R Stat Soc B 1978;40:328-40.

Answers to Study Questions and Quizzes
Q10.1 1. distortion, measure of association, control for, risk factors 2. 1.0 3. 1.0 4. yes 5. yes – The data-based assessment of confounding is made by determining whether the crude estimate of the measure of association is meaningfully different from an adjusted measure of association. Q10.2 1. 0.69 2. 0.65 3. 0.61 4. yes – The data-based assessment of confounding is made by determining whether the crude estimate of the measure of association is meaningfully different from an adjusted measure of association. 5. 0.65, 0.89 – To determine whether there is a meaningful difference in the crude and adjusted estimates based on a specified percent change required between the crude and adjusted estimates, multiply the crude estimate by the specified percent change, and then add and subtract that value to the crude estimated. If the interval obtained contains the adjusted estimate, then there is no meaningful difference. 6. yes 7. no Q10.3 1. True 2. False – The adjusted estimate can be greater or less than the corresponding crude estimate. 3. False – Most epidemiologists prefer to give unequal weight to each stratum specific estimate. Weights are usually determined based on sample size or precision. 4. False – Since confounding is a validity issue, it should not be evaluated by statistical testing, but by looking for a meaningful difference in the crude and adjusted estimates. 5. 1.08 6. 2.17 Q10.4 1. satisfied 2. satisfied 3. not satisfied – Age cannot possibly be associated with blood type. 4. no – Age cannot possibly be associated with blood type. Q10.5 1. 4.00 2. 4.69 3. Black or Hispanic – You might think that the table for the white group should receive more weight since it has a slightly larger sample size, however, the table for the Black or Hispanic group is actually more balanced. See Chapter 14 for a more complete explanation on balanced data. 4. maybe – It depends on whether the investigator considers the difference between 3.59 and 4.16 a meaningful difference. Q10.6 1. confounding, effect measure 2. possible 3. B 4. A 5. D – Data set 3: Recall that the adjusted estimate is a weighted average of the two stratum specific estimates and therefore, must lie between them. Q10.7 1. 4.8% / 3.4% = 1.4 2. 4.4% / 3.4% = 1.3 3. 13.3% / 3.4% = 3.9 4. The two PRs above are the stratumspecific PRs for each of the two gene groups. The three calculated here use one group as a reference and compare the other three to that group. In this example, having low ATH and 2 Aalleles is the reference group compared to those having either one or both of the risk factors (high ATH, 1 B-allele). To

178 Chapter 10. Other Factors Accounted For? Confounding see how these 3 PRs can be used to define two different types of interaction, see the first asterisk on the ActivEpi CD lesson page (10-4) or the box labeled Two Types of Interaction - Additive and Multiplicative. Q10.8 1. True 2. False – Evidence for effect modification is present when the stratum-specific measures of association are different. 3. True 4. effect modification, measles vaccine, measles, age at vaccination

CHAPTER

11

CONFOUNDING CAN BE CONFOUNDING SEVERAL RISK FACTORS
This chapter considers how the assessment of confounding gets somewhat more complicated when controlling for more than one risk factor. In particular, when several risk factors are being controlled, we may find that considering all risk factors simultaneously may not lead to the same conclusion as when considering risk factors separately. We have previously (Chapter 10) argued that the assessment of confounding is not appropriate for variables that are effect modifiers of the exposure-disease relationship under study. Consequently, throughout this chapter, our discussion of confounding will assume that none of the variables being considered for control are effect modifiers (i.e., there is no interaction between exposure and any variable being controlled). Assessing Confounding in the Presence of Interaction
We have restricted our discussion of confounding involving several variables to the situation where none of the variables considered for control are effect modifiers of the exposure-disease relationship under study. This restriction has been made primarily for pedagogical reasons, since it is easier to discuss the confounding among several variables when there is no effect modification. Nevertheless, it is often quite appropriate to consider confounding even when interaction is present. For example, if we are only controlling for one variable, say gender, and we find that the odds ratio for males is 1.3 whereas the odds ratio for females is 3.6 and the crude odds ratio is 10.1, then both confounding and interaction are present and each may be addressed. A similar situation may present itself when two or more variables are being controlled. Moreover, when several variables are being controlled and there is interaction of, say, only one of these variables with the exposure variable, then the remaining variables considered for control may be assessed as potential confounders. For example, if in a cohort study of risk factors for coronary heart disease (CHD), it was determined that cholesterol level (CHL) was the only effect modifier of the exposure variable (say, physical activity level) among risk factors that included age, smoking status, gender and blood pressure, then these latter variables may still be assessed for possible confounding. In the latter situation, one method for carrying out confounding assessment involves stratifying on the effect modifier (CHL) and assessing confounding involving the other variables separately within different categories of CHL. Another approach is to use a mathematical model (e.g., using logistic regression) that contains all risk factors considered as main effects and also contains a product term of exposure with cholesterol. Those risk factors other than CHL can then be assessed for confounding provided the main effect of cholesterol, the exposure variable, and the product of exposure with CHL remains in the model throughout the assessment.

180 Chapter 11. Confounding can be Confounding – Several Risk Factors

Two Important Principles
We have thus far considered only the control of a single confounder in an epidemiologic study. But usually several risk factors are identified and measured for possible control. Recall the a priori criteria for confounding. When several factors meet these criteria, how do we determine which to control for in the analysis? Suppose age and race are two risk factors identified and measured for possible control in a case-control study to assess an exposure disease relationship. It is possible that the adjusted odds ratio, which simultaneously controls for both age and race to give different results from those obtained by controlling for each variable separately. If the odds ratio that controls for all potential risk factors is our standard, then should we always control for all risk factors? Not necessarily. It is possible that only a subset of these factors needs to be controlled to obtain valid results. Suppose these results were obtained from our case-control study:

Here, the odds ratio controlling for age alone is equivalent to the odds ratio controlling for both age and race. In this case, we would not lose anything with regards to validity by selecting only age for control. These examples illustrate two fundamental principles about the control of confounding when several risk factors have been identified and measured. First, the joint (or simultaneous) control of two or more variables may give different results from those obtained by controlling for each variable separately. The adjusted estimate (denoted here as a theta hat, ˆ ) that simultaneously controls for all risk factors under consideration should be the standard on which all conclusions about confounding and the identification of specific confounders must be based. Second, not all the variables in a given list of risk factors may need to be controlled; it is possible that different subsets of such variables can correct for confounding. We will discuss these two principles in the activities that follow.

A Pocket Guide to Epidemiology 181

Study Questions (Q11.1) Suppose age, race, gender, and smoking status are the only risk factors considered for control in assessing an exposure-disease relationship. 1. Describe the adjusted estimate that should be the standard on which all conclusions about confounding and the identification of specific confounders be based. 2. If one fails to consider all potential confounders simultaneously, what might be some of the problems to arise? 3. If the gold standard adjusted estimate controls for all risk factors, is it possible that a subset of such risk factors may also control for confounding? 4. Why might the use of such a subset of variables be advantageous over the use of the adjusted estimate that controls for all potential confounders? Summary There are two fundamental principles about the control of confounding when two or more risk factors have been identified and measured for possible control. 1. The joint or simultaneous control of two or more variables can give different results from those obtained by controlling for each variable separately. 2. Not all variables in a given list of risk factors may need to be controlled. Moreover, depending on the relationships among these risk factors, it is possible that confounding can be corrected by using different subsets of risk factors on the list.

Joint Versus Marginal Confounding
We defined data-based confounding involving a single potential confounder to mean that there is a meaningful difference between the estimated crude effect (which completely ignores a potential confounder) and the estimated adjusted

182 Chapter 11. Confounding can be Confounding – Several Risk Factors effect (which controls for a potential confounder). We now define data-based joint confounding in the presence of 2 or more potential confounders. This occurs when there is a meaningful difference between the estimated crude effect and the estimated adjusted effect, which simultaneously controls for all the potential confounding. Study Questions (Q11.2) Suppose a follow-up study was conducted to evaluate an E D relationship. Age and smoking status were determined as possible control variables. Suppose further that: aRR(age, smoking) = aRR(age) = aRR(smoking) = cRR = 2.4 1.7 1.9 1.5

1. Is this evidence of joint confounding? Why or why not? Suppose for a different follow-up study of the same E D relationship that once again age and smoking status were possible control variables. Suppose further that: aRR(age, smoking) = aRR(age) = aRR(smoking) = cRR = 1.4 2.4 2.4 1.5

2. Is this evidence of joint confounding? Why or why not? In contrast, we define databased marginal confounding to mean that there is a meaningful difference between the estimated crude effect and the estimated adjusted effect that controls for only one of several potential confounders. Study Questions (Q11.2) continued Suppose a follow-up study was conducted to evaluate an E D relationship. Age and smoking status were determined as possible control variables. Suppose that: aRR(age, smoking) = cRR = 2.4 1.5

3. Is there evidence of marginal confounding? Why or why not? 4. If the aRR(age) = 1.4, does this provide evidence of marginal confounding? 5. Does this mean that we should not control for age as a confounder? Joint confounding is the primary criterion for determining the presence of databased confounding when all are eligible for control. Nevertheless, data-based

A Pocket Guide to Epidemiology 183 marginal confounding can help determine whether some potential confounders need not be controlled. Study Questions (Q11.2) continued 6. In the follow-up study described in the previous study question, the: aRR(age, smoking) = 2.4 aRR(age) = 1.5 aRR(smoking) = 2.4 cRR = 1.5 Does this mean that we do not have to control for age? 7. What problem might there be in practice that could prevent the estimate of the effect that controls for all risk factors (e.g., C1, C2, …, Ck)? 8. What should we do if there are too many potential confounders in our list and we are unable to determine the appropriate adjusted estimate? 9. What if the choice of such a subset becomes difficult? Summary Data-based joint confounding occurs when there is a meaningful difference between the estimated crude effect and the estimated adjusted effect that simultaneously controls for all the potential confounders. Data-based marginal confounding occurs when there is a meaningful difference between the estimated crude effect and the estimated adjusted effect, which controls for only one of the several potential confounders. Our conclusions regarding confounding should be based on joint confounding whenever possible. Quiz (Q11.3) Suppose F and G are two distinct risk factors for some disease with dichotomous levels F1, F0, and G1, G0, respectively. The estimated risk ratios describing the association between the disease and some exposure are listed below for various combinations of levels of F and G. Assume that the risk ratio estimates are of high precision (i.e., are based on large sample sizes).
ˆ RR(F1G1 ) 3.0 ˆ RR(F1G 0 ) 3.0 ˆ RR(F G ) 0.3
0 1

ˆ RR(F1 ) 1.0 ˆ RR(F0 ) 1.0 ˆ RR(G ) 1.0
1

ˆ cRR 1.0

ˆ RR(F0 G 0 )

3.0

ˆ RR(G 0 ) 1.0

Determine whether the following statements are True or False. 1. There is evidence of interaction in the data. ??? 2. There is evidence of confounding in the data. ??? 3. At level G0, there is no confounding due to factor F. ??? 4. At level F1, there is no interaction due to factor G. ??? 5. At level F0, there is no interaction and no confounding due to factor G .??? 6. At level G0, there is confounding but no interaction due to factor F ??? 7. It is not necessary to control for either F or G (or both) in order to understand the relationship between D and E. ???.

184 Chapter 11. Confounding can be Confounding – Several Risk Factors

Variable Selection and Control of Confounding
The adjusted estimate that controls for all potential risk factors is the standard on which conclusions about confounding should be based. However, if an adjusted estimate that controls for only a subset of risk factors is equivalent to this standard, we may then choose such a subset for control. Consider a case-control study in which three risk factors, F, G, and H are being considered for control. The crude odds ratio differs from the adjusted odds ratio that controls for all three factors. Because the crude and adjusted estimates differ, we have evidence of data-based joint confounding in these data. Suppose now that controlling for any two of these factors provides the same results as controlling for all three. F, G, and H do not all need to be controlled simultaneously. Controlling for any two of the three risk factors will provide the same results as the standard. We may also wish to consider marginal confounding to see if any single variable is an appropriate subgroup. These results indicate that there is no marginal confounding because each of these results differs from the standard estimate, not one of these variables alone would be an appropriate subgroup for control.

A Pocket Guide to Epidemiology 185 Study Questions (Q11.4)

1. What might be the advantage to controlling for only two of these risk factors rather than all three even though it is the standard? 2. How might you determine which two variables to controls? 3. Why can’t we control for F, G, or H separately? Assume that F, G, H, I, and J are the only risk factors in a case-control study. Suppose further that: cOR aOR(F, G, H, I, J) but aOR(F, G, H, I) = aOR(G, J) = aOR(I) = aOR(F, G, H) = aOR(F, G, H, I, J) and that aOR(any other subset of risk factors) aOR(F, G, H, I, J) Determine whether each of the following is a proper subset of confounders that controls for (joint) confounding in this study by answering Yes or No: 4. 5. 6. 7. {G, J}? {I}? {G, H, I}? {F, G, H}?

Summary When two or more risk factors are considered for control, we can select an appropriate subset of confounders for control. When the results from controlling for various subsets of risk factors are equivalent to the joint control of all risk factors, we can select any one of which provides valid and precise results.

Confounding: Validity versus Precision
The fully adjusted estimate that controls for all factors simultaneously is the standard on which all decisions should be based. Why then would we want to go through the process of seeking out candidate subsets of confounders? If we cannot improve on the validity of the effect measure, why not just use the fully adjusted estimate? The precision of the estimate may justify such an effort. Controlling for a smaller number of variables may yield a

186 Chapter 11. Confounding can be Confounding – Several Risk Factors more precise estimate of effect. The identification of the subset of confounders giving the most precise estimate is important enough to make such examination worthwhile. Consider the following exercise to illustrate this point. Study Questions (Q11.5) A clinical trial was conducted to determine the effectiveness of a particular treatment on the survival of stage 3 cancer patients. The following variables were considered in the analysis: RX = exposure AGE = age at trial entry SERH = serum hemoglobin level TSZ = size of primary tumor INSG = combined index that measures tumor stage and grade. 1. The cRR = 6.28 and the aRR(AGE, SERH, TSZ, INSG) = 8.24. Does this provide evidence of joint confounding in the study? (Assume all quantities above are estimates.) We calculated the aRR for all possible subsets of the four potential confounders. Excluding the crude results and the gold standard, there are 14 possible subsets of these 4 confounders. 2. What criteria may we use to reduce the number of candidate subsets? Below are the results from the gold standard and the 4 candidate subsets whose aRR is within 10% of the gold standard:

3. The most valid estimate results from controlling which covariates? 4. The most precise estimate results from controlling which covariates? 5. Which covariates do you think are most appropriate to control? This exercise has illustrated that we need to consider both validity and precision when assessing an exposure-disease relationship. Getting a valid estimate of effect is most important. Nevertheless, you must also consider the trade-off between controlling for enough risk factors to maintain validity and the possible loss in precision from the control of too many variables. Summary The reason for seeking candidate subsets of all potential confounders is the possibility of improving the precision of the estimated effect. Controlling for fewer variables may (or may not) lead to a more precise estimate of effect.

A Pocket Guide to Epidemiology 187 When controlling for several potential confounders, you should consider the possible trade-offs between: o Controlling for enough risk factors to maintain validity versus o Possible loss in precision from the control of too many variables. Quiz (Q11.6) Suppose that variables F, G, and H have been measured in a certain study and that only F and G are considered to be risk factors for some disease (D). Suppose that it is of interest to describe the relationship between this disease and some study factor (E), and that there is no interaction of any kind present in the data. Finally, suppose that the following relationships hold among various odds ratios computed from the data:

Determine whether the following statements are True or False. 1. There is confounding in the data. ??? 2. Variable F needs to be controlled to avoid confounding. ??? 3. Variable G needs to be controlled to avoid confounding. ??? 4. Variable H needs to be controlled to avoid confounding. ??? 5. Both variables F and G do not need to be controlled simultaneously in order to avoid confounding. ??? A ten-year follow-up study was conducted to determine if someone experiencing food allergies is at increased risk of coronary heart disease. The following covariates were considered in the analysis: AGE = age at enrollment, BMI = body mass index, and SMK = smoking status. The results for the standard and all possible subsets are listed below. The crude risk ratio = 1.02.

6. Is there evidence of confounding? . . . 7. Besides the standard, which are candidate subgroups for control? ???,. ???. 8. Which of the candidate subgroups corresponds to the most valid (including the standard)? . . . 9. Is there more than one candidate subgroup that is the most precise? 10. Which estimate should be used? . . Choices #2 #3 #4 #5 #6 #7 no yes

???. ???, estimate ???. ???. ???.

188 Chapter 11. Confounding can be Confounding – Several Risk Factors

Nomenclature
ˆ

aOR aRR Ci CI cOR cRR D E

Estimated measure of effect Adjusted odds ratio Adjusted risk ratio Confounding variable Confidence interval Crude odds ratio Crude risk ratio Disease Exposure

Reference
Kleinbaum DG, Kupper LL, Morgenstern H. Epidemiologic Research: Principles and Quantitative Methods. John Wiley and Sons Publishers, New York, 1982 (Chapter 14).

Answers to Study Questions and Quizzes
Q11.1
1. The adjusted estimate that simultaneously controls for all 4 risk factors under consideration. 2. Confounding might not be controlled if there is not a subset of potential confounders that yields (essentially) the same adjusted estimate as obtained when all confounders are controlled. 3. Yes, provided the subset yields essentially the same adjusted estimate as the gold standard. 4. Adjusting for a smaller number of variables may increase precision. Also, such a subset provides a more parsimonious description of the exposure-disease relationship. Q11.2 1. Yes, the cRR of 1.5 differs from the aRR(age, smoking) of 2.4 that controls for both potential confounders. 2. No, the cRR of 1.5 is essentially equal to the aRR(age, smoking) of 1.4 that controls for both potential confounders. 3. No, the cRR of 1.5 differs from the aRR(age, smoking) of 2.4, which controls for all potential confounders. This is evidence of joint confounding. 4. No, since the cRR of 1.5 is approximately equal to the aRR(age) of 1.4, there is no evidence of marginal confounding due to age. 5. Not necessarily. Our conclusions regarding confounding should be based on the joint control of all risk factors. 6. Yes. Controlling for smoking alone gives us the same result as controlling for both risk factors. We might still wish to evaluate the precision of the estimates before making a final conclusion. 7. There may be so many risk factors in our list relative to the amount of data available that the adjusted estimate cannot be estimated with any precision at all. 8. Then we may be forced to make decisions by using a subset of this large initial set of risk factors. 9. The use of marginal confounding may be the only alternative. Q11.3 1. True – There is interaction because the risk ratio estimated in one stratum (F0G1) is 0.3, which is quite different from the stratum-specific risk ratios of 3.0 in the other strata. 2. True – The presence of strong interaction may preclude the assessment of confounding. Also, the value of an adjusted estimate may vary depending on the weights chosen for the different strata. 3. False – The RR for F1 and F0 at level G0 are both 3.0. These differ from the overall RR at level G0 of 1.0.

A Pocket Guide to Epidemiology 189
Therefore, at level G0, there is confounding due to factor F. 4. True 5. False – There is interaction and possibly confounding. At level F0, the RR for G1 and G0 are very different, and both are very different from the overall risk ratio at level F0. 6. True 7. False – Both confounding and interaction are present and each should be addressed. Q11.4 1. Controlling for fewer variables will likely increase the precision of the results. 2. The two that provide the most precise adjusted estimate. 3. Controlling for any of these three factors alone yields different results than controlling for all three, which is the standard on which our conclusions should be based. 4. Yes 5. Yes 6. No 7. Yes Q11.5 1. Yes, the cRR differs from the aRR controlling for all potential confounders, which is the gold standard. 2. We may choose to only consider those results within 10% of the gold standard. In this case, that would be 8.24 ± 0.82 which is a range of values between 7.42 and 9.06. 3. Controlling for all the covariates provides the most valid estimate. It is the gold standard. 4. Controlling for both INSG and AGE provides the narrowest confidence interval and hence is the most precise. 5. Debatable: Controlling for SERH alone yields an almost identical aRR as the gold standard, increases precision, and is the stingiest subset. Controlling for INSG and AGE provides a slightly larger increase in precision (than controlling for SERH only) and its aRR is within 10% of the standard. Consider the trade-off between parsimony and political/scientific implications of not controlling for all risk factors, and more precision from controlling for fewer risk factors. Q11.6 1. True 2. True 3. False – Variable G does not need to be controlled since aOR(F,G)=aOR(F). In other words, controlling for F alone yields the same results as the gold standard, controlling for both F and G. 4. False – Variable H is not a risk factor in this study, and therefore should not be considered a confounding. 5. True 6. Yes – The cRR of 1.02 differs from the standard RR of 4.10 that controls for all potential confounders. 7. #3, #5, #7 8. #1 – The most valid estimate controls for all risk factors measured. 9. Yes – Candidate subgroups 1 and 7 are equally precise. 10. #1 – The gold standard is the most valid estimate; has the same precision as obtained for candidate 7. No precision is gained by dropping any risk factors so it can be argued the gold standard is the ‘political’ choice for it controls for all considered risk factors. Controlling only for SMK is the best choice for it gives the smallest, most precise subset of variables.

CHAPTER

12

SIMPLE ANALYSES2 2 TABLES ARE NOT THAT SIMPLE
This chapter discusses methods for carrying out statistical inference procedures for epidemiologic data given in a simple two-way table. We call such procedures simple analyses because we are restricting the discussion here to dichotomous disease and exposure variables only and we are ignoring the typical analysis situation that considers the control of other variables when studying the effect of an exposure on disease.

WHAT IS SIMPLE ANALYSIS?
When analyzing the crude data that describes the relationship between a dichotomous exposure and dichotomous disease variable, we typically want to make statistical inferences about this relationship. That is, we would like to determine whether the measure of effect being estimated is statistically significant and we would like to obtain an interval estimate that considers the sample variability of the measure of effect. The tables shown here have been described in previous chapters to illustrate data from three different studies, a cohort study to assess whether quitting smoking after a heart attack will reduce one 's risk for dying, a case-control study to determine the source of an outbreak of diarrhea at a resort in Haiti, and a person-time cohort study to assess the relationship between serum cholesterol level and mortality. In each study, a measure of effect was computed to estimate the extent of the relationship between the exposure variable and the health outcome variable. In the quit smoking cohort study, the effect measure was a risk ratio and its estimate was 2.1. In the outbreak study, the effect measure was an odds ratio and its estimate was 3.2. And in the cholesterol mortality study, the effect measure was a rate ratio, also called an incidence density ratio, and its estimate was 3.5. We have discussed how to interpret each of these estimates in terms of the exposure disease-relationship being studied. All three estimates, even though

192 Chapter 12. Simple Analyses – 2x2 Tables are Not That Simple dealing with different study types and different study questions, are similar in that they are all larger than the null value of one, and they all indicate that there is a moderately large effect from exposure. Nevertheless, we must be careful to realize that each of these three estimates is based on sample data. If a different sample had been drawn, any of these estimates might have resulted in a different value, maybe a lot larger, maybe closer to the null value of one. That is, there is always random error associated with any sample estimate. We call these estimates point estimates because they each represent a single number or point from the possibly wide range of numbers that might have been obtained if different samples had been drawn. So, we might wonder, given the inherent variability in a point estimate, how can we draw conclusions about the population parameters being estimated? For example, in the first cohort study, what can we say about the population risk ratio based on the estimated risk ratio? Or, in the case-control study, what can we conclude about the population odds ratio based on the estimated odds ratio? In the person-time study what can we conclude about the population rate ratio based on the estimated rate ratio? In answering these questions, we typically have one of two objectives. We may want to determine whether we have evidence from the sample that the population risk ratio, odds ratio or rate ratio being estimated is different from the null value of one. For this objective, we use hypothesis testing. Or, we may want to determine the precision of our point estimate by accounting for its sampling variability. For this objective, we use interval estimation. The methods used to achieve these objectives comprise the general subject matter of statistical inference. When our attention is focused on the relatively simple situation involving only one dichotomous exposure variable and one dichotomous disease variable, as illustrated by these three studies, we call these methods simple analyses. Summary Estimates of measures of effect such as RR, OR, and IDR are point estimates, since each estimate represents a single number that may vary from sample to sample. Statistical inference involves drawing conclusions about the value of a measure of effect in a population, based on its estimate obtained from a sample. The two types of statistical inference procedures are hypothesis testing and interval estimation.

STATISTICAL INFERENCES – A REVIEW
The activities in this section review fundamental concepts and methods of statistics. Our primary focus concerns how to draw conclusions about populations based on data obtained in a sample. We assume that you already have some previous exposure to basic statistical concepts, including the distinction between a sample and a population, a sample statistic and a population parameter, some important distributions like the normal, binomial, Student 's t, and chi square distributions. We also assume that you have some previous exposure to

A Pocket Guide to Epidemiology 193 the concepts underlying statistical tests of hypothesis and confidence intervals, which are the two types of statistical inferences possible. Our focus here will be to review statistical inference concepts in the context of the statistical questions that apply to the analysis of a 2 x 2 table, which is the kind of data we are considering in a simple analysis. You may wish to skip this entire review section and proceed to the next section, Cohort Studies, on page 12-4. We begin by using data from a famous "incident" to distinguish between the two types of statistical inference procedures: hypothesis (significance) testing and confidence interval estimation. See if you can guess what "incident" we illustrate.

Statistical Inference Overview
Here are some data from an incident in which a group of persons were at risk of dying. From these data, we can find the proportions who died for men and women, separately. We can see that 79.7% of the men died, but only 25.6% of the women died. Clearly these two percentages are meaningfully different since the men had a much higher risk for dying than the women. But can we also claim that there is a difference in the risk for dying among men and women in the population from which these samples came? In other words, is the difference in the risks for men and women statistically significant? If we wish to draw conclusions about a population from data collected from a sample, we must consider the methods of statistical inference. In particular, we must view the two proportions or percentages as estimates obtained from a sample. ˆ ˆ Let 's focus on the two sample proportions, which we denote p M and p W . The corresponding population proportions are denoted

p M and p W , without “hats”.

Statistical inference draws conclusions about a population parameter based on information obtained from a sample statistic. So, what is the population parameter considered for these data and what is its corresponding sample statistic?

Since our focus here is to compare the proportions for males and females, one logical choice for our parameter of interest is the difference between the two

194 Chapter 12. Simple Analyses – 2x2 Tables are Not That Simple population proportions. The corresponding sample statistic is the difference between the two estimated proportions. Study Questions (Q12.1) 1. What other (epidemiologic) parameters could also be considered as alternatives to the difference in the two proportions? Hypothesis testing can be used here to determine whether the difference in the two proportions is statistically significant. This is one of the two types of statistical inference questions we may ask. Our hypothesis in this case, usually stated as what we want to disprove, is that the true difference in the two population proportions is zero. This is called the null hypothesis. In hypothesis testing, we seek evidence from the sample to disprove the null hypothesis in favor of the alternative hypothesis that there is a difference in the population proportions. Study Questions (Q12.1) continued 2. If the parameter of interest is the risk ratio (RR), how would you state the null hypothesis? 3. If the parameter of interest is the odds ratio (OR), how would you state the null hypothesis? We can use interval estimation to determine the precision of our point estimate. Here, our goal is to use our sample information to compute two numbers, say, L and U, that define a confidence interval for the difference between the two population proportions. Using a confidence interval, we can predict with a certain amount of confidence, say 95%, that the limits, L and U, bound the true value of the parameter. For our data, it turns out, that the lower and upper limits for the difference in the two proportions are .407 and .675, respectively. It may appear from these two numbers that an interval estimate is less precise than a point estimate. The opposite is actually true. The range of values specified by the interval estimate actually takes into account the unreliability or variance of the point estimate. It is therefore more precise, since it uses more information to describe the point estimate. In general, interval estimation and hypothesis testing can be contrasted by their different approaches to answering questions. A test of hypothesis arrives at an answer by looking for rare or unlikely sample results. In contrast, interval estimation arrives at its answer by looking at the most likely results, that is, those values that we are confident lie close to the parameter under investigation.

A Pocket Guide to Epidemiology 195

Summary Statistical inference concerns drawing conclusions about a population parameter based on information obtained from a sample statistic. The two types of statistical inference are hypothesis testing and interval estimation. When we test a hypothesis, we typically want to disprove a null hypothesis. When doing interval estimation, we want to obtain a confidence interval that provides upper and lower limits that we can be, say 95%, confident covers the population parameter of interest. A test of hypothesis looks for rare or unlikely results from our sample, where a confidence interval looks for likely results.

The Incident
The story of the Titanic is well known. The largest ship that had ever been built up to that time, she left Southampton, England on her maiden voyage to New York on Wednesday, April 10, 1912, carrying many of the rich and famous of England and the United States, but also many of more modest means. Because the Titanic was so large and so modern, many thought that she could not sink. After a stop at Cherbourg France, where she took on many 3rd class passengers seeking new lives in the New World, and a brief stop off Queenstown, Ireland, she set out across the Atlantic. At 11:40 on the evening of April 14th, the Titanic struck an iceberg and, by 2:15 the next morning, sank. Of 2,201 passengers and crew, only 710 survived. Some facts can be gleamed about the passengers, about who survived and who did not. One underlining question of interest in any disaster of this sort is did everyone have an equal chance of surviving? Or, stated in statistics terms, was the probability of surviving independent of other factors.

Hypothesis Testing
We illustrate how to carry out a statistical test of hypothesis to compare survival of men and women passengers on the Titanic. We want to assess whether the difference in sample proportions for men and

196 Chapter 12. Simple Analyses – 2x2 Tables are Not That Simple women is statistically significant. A test of hypotheses, also called a test of significance, can be described as a seven-step procedure. In step one we state the information available, the statistical assumptions, and the population parameter being considered. In our example, the information includes the numbers of men and women passengers and the sample proportions. We need to assume that the data represents a sample from a larger population and that each sample proportion is approximately normally distributed. The population parameter of interest is the difference between the two population proportions being estimated (PM - PW). The corresponding sample ˆ ˆ statistic is the difference between the two estimated proportions ( P M - P W ) . In step 2, we specify the null and alternative hypotheses. A hypothesis is a claim about a value of a population parameter. The hypothesis that we plan to test is commonly called the null hypothesis. This is often the accepted state of knowledge that we want to question. The null hypothesis in our example is that the difference in the two population proportions is zero. H0 is essentially treated like the defendant in a trial. It is assumed true, or innocent, until the evidence from the data makes it highly unlikely to have occurred by chance. Our testing goal here is to see if we have evidence to disprove this null hypothesis. The alternative hypothesis, typically called HA, gives the values the parameter may take if the null is false. In our example, the alternative hypothesis is that the difference in population proportions is not equal to zero. This is called a two-sided alternative because it states that we are interested in values both above and below the null. The alternative hypothesis would be called one-sided if, before looking at our data, we were interested in determining only whether men were at greater risk for dying than women. To avoid biasing one 's analysis, both the null and alternative hypothesis should be made without looking at the study data and be based only on the a priori objectives of the study. In step 3, we specify the significance level, alpha. We will set the significance level for our example at .05 or 5 percent. This means that in carrying out our procedure, we are willing to take a 5 percent risk of rejecting the null hypothesis even if it is actually true. Equivalently, the significance level tells us how rare or unlikely our study results have to be under the null hypothesis in order for us to reject the null hypothesis in favor of the alternative hypothesis. An alpha of 5 percent means that, if the null hypothesis is actually true, we will have a 5% chance of rejecting it. Summary of first 3 steps. A statistical test of hypothesis can be described as a seven-step procedure. The first three steps are: Step 1: State the information available, the statistical assumptions, and the population parameter being considered.

A Pocket Guide to Epidemiology 197 Step 2: Specify the null and alternative hypotheses. Step 3: Specify the significance level alpha ( ). In step 4 of our hypothesis testing procedure, we must select the test statistic to use, and we must state its sampling distribution under the assumption that the null hypothesis is true. Because the parameter of interest is the difference between two proportions, the test statistic T is given by the difference in the two sample proportions divided by the estimated standard error of this sample difference under the null hypothesis. The denominator here is computed using an expression ˆ involving the pooled estimate, p , of the common proportion for both groups that would result under the null hypothesis.
Step 4: Select the test statistic and state its sampling distribution under H0.

T

ˆ pM ˆ ˆ p(1 p)(

ˆ pW 1 nM 1 ) nW

ˆ p

ˆ nM p M nM

ˆ nW pW nW

Study Question (Q12.2) The pooled estimate of the common proportion for two groups is a weighted average of the two sample proportions, where the weights are the sample sizes used to compute each proportion. The sample size proportions and their corresponding sample sizes are .7972 and 1667, respectively for men, and .256 and 425 for women. 1. Compute the pooled estimate from the above information. (You will need a calculator to obtain your answer.) The sampling distribution of this test statistic is approximately the standard normal distribution, with zero mean and unit standard deviation, under the null hypothesis. In step 5, we formulate the decision rule that partitions the possible outcomes of the test statistic into acceptance and rejection regions. Because our test statistic has approximately the standard normal or Z distribution under the null hypothesis, the acceptance and rejection regions will be specified as intervals along the Z-axis under the curve of this distribution. In particular, because our alternative hypothesis is two-tailed and since our significance level is .05, these two regions turn out to be as shown here by the red and green lines. (Note: the red lines are in the tail areas < -1.96 and > 1.96; the green line between –1.96 and 1.96.) The area under the standard normal curve above the interval described as the acceptance region is .95 .The area under the curve in each tail of the distribution identified as rejection regions is .025. The sum of these two areas is .05, which is our chosen significance level. The -1.96 on the left side under the curve is the 2.5 percentage point of the standard normal distribution, and the 1.96 on the right side under the curve is the 97.5 percentage point.

198 Chapter 12. Simple Analyses – 2x2 Tables are Not That Simple

Our decision rule can now be described as follows. If the value of the test statistic T computed from our data falls into the rejection region, we reject the null hypothesis in favor of the alternative hypothesis. However, if the observed study value falls into the acceptance region, we do not reject the null hypothesis. Step 6 of our process simply requires us to compute the value of the test statistic T from the observed data. We will call the computed value T* to distinguish it from the test statistic T. Here again are the sample results:
Step 6: Compute the test statistic T from the data

T

ˆ pM ˆ ˆ p(1 p)(

ˆ pW 1 nM 1 ) nW

ˆ p

ˆ nM p M nM

ˆ nW pW nW

Computed T = T* ˆ p .6874 (men and women combined)

ˆ pM ˆ pW

.7972 nM .2565 nW

1667 (men) 425 (women)

Substituting the sample information into the formula for T, our computed value T* turns out to be 21.46.

Finally, in Step 7, we use our computed test statistic to draw conclusions about our test of significance. In this example, the computed test statistic falls into the extreme right tail of the rejection region because it is much larger than 1.96.

A Pocket Guide to Epidemiology 199 Consequently, we reject the null hypothesis and conclude that we have a statistically significant difference between the two proportions at the .05 significance level. We therefore conclude that of all those that could have been aboard the Titanic, men were more likely to die than women. Summary A statistical test of hypothesis can be described as a seven-step procedure. The first three steps are: Step 1: State the information available, statistical assumptions, and the population parameter being considered. Step 2: Specify the null and alternative hypotheses. Step 3: Specify the significance level alpha ( ). Step 4: Select the test statistic and state its sampling distribution under the null hypothesis. Step 5: Formulate the decision rule in terms of rejection and acceptance regions under the null hypothesis. Step 6: Compute the test statistic using the observed data. Step 7: Draw conclusions, i.e., reject or do not reject the null hypothesis at the alpha significance level.

Hypothesis Testing – The P-value
We have found that the difference in the sample proportions of the men and women who died on the Titanic was statistically significant at the 0.05 significance level. In particular, the computed value of the test statistic fell into the extreme right tail of the rejection region. This tells us that if the null hypothesis were true, the observed results had less than a 5% chance of occurring. That is, the results were quite unlikely under the null hypothesis. We may wonder, then, exactly how unlikely, or how rare, were the observed results under the null hypothesis? Were they also less than 1% likely, or less than 0.1% likely, or even rarer? The answer to these questions is given by the P-value. The P-value gives the probability of obtaining the value of the test statistic we have computed or a more extreme value if the null hypothesis is true. Let 's assume, as in our example, that the test statistic has the standard normal distribution under the null hypothesis. To obtain the P-value, we must determine an area under this curve. Here, we show four different areas that correspond to where the computed value T* falls under the curve and to whether the alternative hypothesis is one-sided or two-sided.

200 Chapter 12. Simple Analyses – 2x2 Tables are Not That Simple If the alternative hypothesis is an upper one-sided hypothesis, then the P-value is the area under the curve to the right of T* (upper right distribution in the above figure). If the alternative hypothesis is a lower one-sided hypothesis, then the Pvalue is the area under the curve to the left of T* (upper left distribution in the above figure). If the alternative hypothesis is two-sided and T* falls in the right tail under the curve, then the P-value is the sum of the areas under the curve to the right of T* and to the left of -T*. If the alternative hypothesis is two-sided and T* falls in the left tail under the curve, then the P-value is the sum of the areas under the curve to the left of T* and to the right of -T*. The P-value gives the area under the curve that shows the probability of the study results under the null hypothesis. Study Questions (Q12.3) 1. Which of the four scenarios above correspond to the P-value for our Titanic example? (Hint: T* = 12, HA is two-sided.) 2. To obtain the P-value for a 2-sided HA, why is it not necessary to compute 2 areas under the normal curve? Now, let 's see how rare our computed test statistic is under the null hypothesis. The computed test statistic is 12.0, so we need to find the area under the normal curve to the right of the value 12.0, and to the left of -12.0. One way to determine this area is to use a table of the percentage points of the standard normal or Z distribution. In one such table, as illustrated in the figure that follows this paragraph, the highest percentage point is 3.8, corresponding to the 99.99 percentage point. Although we can 't find the area to the right of 12.0 under the normal curve exactly, we can say this area is less than .0001, clearly a very small value.

Study Questions (Q12.3) continued 3. If our alternative hypothesis had been one-tailed, what would be our P-value? 4. Since our alternative hypothesis was actually two-tailed, what is our P-value? 5. Based on your answer to question 2, has a rare event occurred under the null hypothesis? 6. Based on the P-value here, what should you conclude about whether or not the test of hypothesis is significant? P-values are often used as an alternative way to draw conclusions about a test of hypothesis rather than specifying a fixed significance level in advance of

A Pocket Guide to Epidemiology 201 computing the test statistic. If the P-value is small enough, so that a rare event has occurred, then we reject the null hypothesis. If the P-value is not small, then we would not reject the null hypothesis. So, how small must the P-value be for our results to be considered rare? The answer here essentially depends on the alpha ( ) significance level we wish to use. A conventional choice for alpha is 0.05, although a frequent alternative choice is 0.01. Thus if the P-value is .01? b) P = .023? 10. c) P < .001? d) .01 < P = .05? e) P = .0002? Summary The P-value describes how unlikely, or how rare, are the observed results of one’s study under the null hypothesis. For one-tailed alternative hypotheses, the P-value is determined by the area in the tail of the distribution, beyond the computed test statistics (i.e., to the right or left), under the null hypothesis. For two-tail alternative hypotheses, the P-value is twice the area in the tail of the distribution, beyond the computed test statistic, under the null hypothesis. The P-value is often used as an alternative way to draw conclusions about a null hypothesis rather than specifying a significance level prior to computing the test statistics. If the P-value is considered small by the investigators, say, less than .05, .01, we reject the null hypothesis in favor of the alternative hypothesis. If the P-value is not considered small, usually greater than .10, we do not reject the null hypothesis. Z-scores and Relative Frequencies – The Normal Density Function Most statistics texts include a table that lets you relate z-scores and relative frequencies in a normal density. The tables always give this information for the Standard Normal Density, so that the x-axis of the density is marked out in zscores. The normal density tool we have been working with provides the same information more easily. For example, to find the relative frequency of values with z-scores below -1.5, just drag the left flag to the z-score value -1.5 and read the relative frequency in the lower left box, 0.067.

202 Chapter 12. Simple Analyses – 2x2 Tables are Not That Simple

Summary The Standard Normal Density curve, for which tables are usually given in statistics textbooks, relates z-scores and relative frequencies. The Normal Density tool (lesson page 12-2 of the ActivEpi CD ROM) provides the same information and gives you some practice exercises. Quiz (Q12.4) Fill in the Blanks. 1. We use ??? to assess whether the population parameter is different from the null value. 2. When determining the precision of a point-estimate, ??? accounts for sampling variability. 3. ??? looks for rare or unlikely results. 4. By looking at the most likely results, ??? finds those values that we are confident lie close to the population parameter. Choices: hypothesis testing interval estimation

5. The ??? gives the risk we are willing to take for rejecting the null hypothesis when the null hypothesis is false. 6. The ??? can be either upper-one-sided, lower one-sided or two-sided. 7. If the computed value of the test statistic falls into the ???, we reject the ??? and conclude that the results are ??? significant. Choices acceptance region alternative hypothesis meaningfully null hypothesis rejection region significance levels statistically 8. The ??? describes how rare or how unlikely are the observed results of one 's study under the ???. 9. If the P-value satisfies the inequality P>.30, we should ??? the null hypothesis. 10. If the P-value satisfies the inequality P1, since prior to looking at the data the investigators were interested in whether continuing smoking was more likely than quitting smoking to affect mortality.

Because the risk ratio is the ratio of cumulative incidences for the exposed group (CI1), divided by cumulative incidences for the unexposed group (CI0), we can equivalently state the null hypothesis in terms of the difference in population cumulative incidences as shown here: Because the risk ratio equals one if and only if the risk odds ratio equals one, we can also equivalently state the null hypothesis in terms of the risk odds ratio. Because cumulative incidence is a proportion, the cumulative incidence version of the null hypothesis implies that our test about the risk ratio is equivalent to testing a hypothesis about the difference between two proportions: H0: p1 – p0 = 0. The test statistic is the difference in the two estimated cumulative incidences divided by the estimated standard error of this difference, under the null hypothesis that the risk ratio is one. Because the sample sizes in both groups are reasonably large, this test statistic has approximately a standard normal distribution under the null hypothesis. The computed value of the test statistic is obtained by substituting the estimated cumulative incidences and corresponding sample sizes into the test statistic formula as shown here. The resulting value is 2.65. The P-value for this test is then obtained by finding the area in the right tail of the standard normal distribution above the computed value of 2.65. The exact Pvalue turns out to be .0040. Because the P-value of .0040 is well below the conventional significance level of .05, we reject the null hypothesis and conclude that the risk ratio is significantly greater than the null value of one. In other words, we have found that among heart attack patients who smoke, continuing smokers have a significantly higher risk for dying than smokers who quit after their heart attack. Summary When testing the hypothesis about a risk ratio (RR) in a cumulative-incidence cohort study, the null hypothesis can be equivalently stated as either RR = 1, CI1 – CI2 = 0, or ROR = 1, where CI1 and CI2 are the cumulative incidences for the exposed and unexposed groups, respectively.

208 Chapter 12. Simple Analyses – 2x2 Tables are Not That Simple The alternative hypothesis can be stated in terms of the RR either as RR 1, RR > 1, or RR < 1 depending on whether the alternative is two-sided, upper one-sided, or lower one-sided, respectively. To test the null hypothesis that RR = 1, the test statistic is the same as that used to compare the difference between two proportions. Assuming large samples, the test statistic has approximately the N(0,1) distribution under H0.

Chi Square Version of the Large-sample Test
The large-sample test for a risk ratio can be carried out using either the normal distribution or the chi square distribution. The reason for this equivalence is that if a standard normal variable Z is squared, then Z square has a chi square distribution on 1 degree of freedom. More specifically, for our mortality study of heart attack patients, here is the test statistic that we previously described, it follows a standard normal distribution under the null hypothesis that the risk ratio equals 1. The square of this statistic is shown next to its corresponding chi square distribution:

With a little algebra, we can rewrite the statistic in terms of the cell frequencies a, b, c and d of the general 2 by 2 table that summarizes the exposure disease information in a cohort study that estimates cumulative incidence.

For the mortality study of heart attack patients, the values of the cell frequencies are shown following this paragraph. Substituting these values into the chi square statistic formula, we obtain the value 7.04. This value is the square of the computed test statistic we found earlier (2.652 7.04)

A Pocket Guide to Epidemiology 209

Is the chi square version of our test significant? The normal distribution version was significant at the .01 significance level, so the chi square version had better be significant. But in comparing these two versions of the test, we need to address a small problem. A chi-square statistic, being the square of a Z statistic, can never be negative, but a standard normal statistic can either be positive or negative. Large values of a chi square statistic that might indicate a significant result could occur from either large positive values or large negative values of the normal statistic. In other words, if we use a chi square statistic to determine significance, we are automatically performing a test of a two-sided alternative hypothesis even though we are only looking for large values in the right tail of the distribution. Study Questions (Q12.8) The .99 and .995 percentage points of the chi square distribution with 1 df are given by the values 6.635 and 7.789, respectively. 1. Does the computed chi square value of 7.04 fall in the upper 1 percent of the chi square distribution? 2. Does the computed chi square value of 7.04 fall in the upper .5 percent of the chi square distribution? 3. Would a test of a two-sided alternative for RR be significant at the .01 significance level? (Note: The computed test statistic is 7.04.) 4. Would a test of a two-sided alternative for RR be significant at the .005 significance level? So, if our chi square statistic allows us to assess a two-sided alternative hypothesis about the risk ratio, how can we assess a one-sided alternative? One way is simply to carry out the normal distribution version of the test as previously illustrated. The other way is to divide the area in the right tail of the chi square curve in half. Study Questions (Q12.8) continued

210 Chapter 12. Simple Analyses – 2x2 Tables are Not That Simple 5. Using the above results, would a test of an upper one-sided alternative RR > 1 be significant at the .01 significance level? 6. Would a test of an upper one-sided alternative RR > 1 be significant at the .005 significance level? Summary An alternative but equivalent way to test for the significance of a risk ratio is to use a chi square test. The square of a standard normal variable has the chi square distribution with one degree of freedom. We can directly compute the chi square statistic using a formula involving the cell frequencies of a 2 x 2 table for cohort data that allows for estimation of risk. When we use a chi square statistic to determine significance, we are actually performing a test of a two-sided alternative hypothesis. We can assess a one-sided alternative either using the normal distribution version of the test or by using the chi square distribution to compute a P-value.

The P-value for a One-Sided Chi Square Test
We now describe how to compute a P-value when using a chi square test involving a one-sided alternative hypothesis about a risk ratio. We will illustrate this computation once again using data from the mortality study on smoking behavior of heart attack patients. To determine the P-value, we must first compute the area shaded as pink (lighter shaded in the right tail of the distribution) under the chisquare distribution above the value of the computed chisquare statistic 7.04. This area actually gives the P-value for a 2-sided alternative hypothesis. In fact, it is equivalent to the combined area in the two tails of the corresponding normal distribution defined by the computed value of 2.65. For the upper one-sided alternative that the population risk ratio is greater than 1, we previously found the P-value to be the area only in the right tail of the normal curve above 2.65. This right tail area is but one-half of the corresponding area in the right tail of the chi-square curve above 7.04. Consequently, the area in the right tail of the chi square curve gives twice the P-value for a one-tailed alternative. Therefore, this area must be divided by 2 in order to get a one-sided P-value. Study Questions (Q12.9) 1. Suppose the alternative hypothesis in our example above was a lower onesided alternative and the computed value of the chi square statistic is 7.04, corresponding to a computed Z statistic of +2.65. What is the corresponding P-value?

A Pocket Guide to Epidemiology 211 2. Based on your answer to the previous question, what would you conclude about the null hypothesis that the RR equals 1? 3. Suppose the alternative hypothesis was a lower one-sided alternative and the computed value of the chi square statistic is 7.04, but this time corresponding to a computer Z statistic of –2.65. What is the corresponding P-value? 4. Based on your answer to the previous question, what would you conclude about the null hypothesis that the RR equals 1? Summary When using a chi square test of a two-sided alternative hypothesis about a risk ratio, the P-value is obtained as the area under the chi square curve to the right of the computed test statistic. If the alternative hypothesis is one-sided, e.g., RR > 1, then the P-value is onehalf the area in the right tail of the chi square curve.

Testing When Sample Sizes Are Small
This table displays hypothetical results from a five-month randomized clinical trial comparing a new anti-viral drug for shingles with the standard anti-viral drug “Valtrex”. Only 13 patients were involved in the trial, which was a pilot study for a larger clinical trial. The estimated risk ratio from these data is 3.2, which indicates that, in the sample of 13 patients, the new drug was 3.2 times more successful than the standard drug. Is this risk ratio significantly different from the null value of one? We are considering a small sample size here, so a large-sample test of hypothesis is not appropriate. (Note: How large is large is somewhat debatable here, but it is typically required that the sample size for a proportion must be large enough (e.g., >25) for the sample to be normally distributed.) However, there is a statistical test for sparse data, called Fisher 's Exact Test, that we can use here. Fisher 's Exact Test is appropriate for a 2x2 table relating a dichotomous exposure variable and a dichotomous disease variable. To use Fisher 's exact test, we must assume that the values on the margins of the tables are fixed values prior to the start of the study. If we make this 'fixed marginals assumption ', we can see that once we identify the number in anyone cell in the table, say the exposed cases or a cell, the numbers in the other three cells can be determined by using only the frequencies on the marginals. Study Questions (Q12.10) 1. Determine the formulas for calculating the values for b, c, and d in terms of a and the fixed marginal values.

212 Chapter 12. Simple Analyses – 2x2 Tables are Not That Simple

2. Calculate the values in the other cells of the table above knowing only the marginal values and the value in one cell. To test our hypothesis about the risk ratio, we need only consider the outcome for one of the four cells of the table. For example, we would like to know what values we might get in the a cell that would be unlikely to occur under the null hypothesis that the risk ratio, or for that matter the risk odds ratio, equals 1. In particular, we would like to determine whether the value obtained in the a cell for our study, which turned out to be 4, is a rare enough event under the null hypothesis for us to reject the null hypothesis and conclude that the observed risk ratio is statistically significant. We can answer this question by computing the Pvalue for our test. Study Questions (Q12.10) continued

3. Suppose the a cell value was 5 instead of 4, assuming the marginals are fixed. What would be the corresponding revised values for b, c, and d? 4. What would be the risk ratio for the revised table? 5. Is the revised risk ratio further away from the null risk ratio than the risk ratio of 3.2 actually observed? 6. Are there any other possible values for the a cell that would also be further away from the null risk ratio than 3.2? 7. Based on the previous questions, why would we want to compute the probability of getting an a cell value of 4 or 5 under the null hypothesis? To compute the P-value for Fisher 's Exact test, we need to determine the probability distribution of the a cell frequency under the null hypothesis. Assuming fixed-marginals, this distribution is called the hypergeometric distribution. The formulae for the hypergeometric distribution and the corresponding P-value for Fisher 's Exact Test are described in an asterisk on Lesson Page 12-4 (cohort studies) and 12-6 (case-control studies) of the ActivEpi CD ROM. Study Questions (Q12.10) continued Using the hypergeometric distribution, the (Fisher’s Exact Test) one-sided P-value for the study described in this section is P(a =4 or 5|Rr=1) = P(a=4|RR=1) + P(a=5|RR=1) = .0816 + .0046 = .0863. 8. What are your conclusions about the null hypothesis that RR = 1?

A Pocket Guide to Epidemiology 213 Summary Fisher’s Exact Test provides a test of significance for a risk ratio or an odds ratio when the data are sparse. To use Fisher’s exact test, we must assume that the values on the margins of a 2x2 table are fixed values prior to the start of the study. The marginal frequencies of a 2x2 table provide no information concerning the strength of the association. To compute the P-value for Fisher’s Exact test, we need to determine the probability distribution of the a cell frequency under the null hypothesis. Assuming fixed-marginals, the a cell has the hypergeometric distribution. Computer programs, including a DataDesk program for ActivEpi, are available to calculate the P-value for this test.

COHORT STUDIES INVOLVING RISK RATIOS
(continued) Large-sample version of Fisher 's Exact Test - The MantelHaenszel Test
Let 's consider again the mortality study data on smoking behavior of heart attack patients. For these data, we have previously described a large-sample chi square statistic for testing the null hypothesis that the risk ratio is 1.

Because the mortality data involves a large-sample, we do not need to use Fisher 's Exact Test for these data. Nevertheless, we could still compute the Fisher 's Exact test statistic. To compute Fisher 's Exact test, we assume that the frequencies on the margins of the table are fixed and then we compute the probability under the null hypothesis of getting an a cell value at least as large as the value of 27 that was actually obtained. We would therefore compute and sum 15 probability values, from a= 27 to a= 41 to obtain the P-value for Fisher 's Exact Test. Although such a calculation can be accomplished with an appropriate computer program, a more convenient large-sample approximation to Fisher 's Exact Test is often used instead when the cell frequencies in the two by two table are moderately large. This large-sample approach is called the Mantel-Haenszel (MH) test for simple analysis. The Mantel-Haenszel statistic is shown below. This statistic has an approximate chi square distribution with one degree of freedom under the null hypothesis that the risk ratio is 1. Consequently, the P-value for a one-sided alternative using this

214 Chapter 12. Simple Analyses – 2x2 Tables are Not That Simple statistic is obtained in the usual way by finding the area under the chi-square distribution above the computed test statistic and then dividing this area by 2. If we compare the large-sample chi square statistic for Fisher 's exact test with the large-sample chi square statistic previously described for comparing two proportions, we see that these two test statistics are remarkably similar. In fact, they differ only in that the statistic for approximating Fisher Exact Test contains n-1 in the numerator but the earlier large-sample version contains n in the numerator. When n is large, using either n or n-l in the numerator will have little effect on the computed chi square statistic. In our mortality data example, for instance, the value of the chi square approximation to Fisher 's exact test is equal to 6.99. The large-sample chi square statistic for comparing two proportions was previously shown to be 7.04. Clearly, the two chi square statistics are very close, although not exactly equal. The corresponding one-sided P-values are .0040 and .0041, respectively, essentially equal.

*Note: in the box above one-sided p-values are provided.

This example illustrates that in large-samples, these two chi square versions are essentially equivalent and will lead to the same conclusions about significance. Study Questions (Q12.11)

For the above data, we previously showed that the one-sided P-value for Fisher’s Exact test is .0863. The MH statistic computed from these data turns out to be 3.46. 1. The P-value for a two-sided MH test is .0630. What is the P-value for a onesided Mantel-Haenszel test? 2. Why is the P-value for the Mantel-Haenszel test different from the P-value for Fisher’s Exact test?

A Pocket Guide to Epidemiology 215 3. The computed value for the large-sample chi square statistic for comparing two proportions is 3.75. Why is this latter value different from the computed Mantel-Haenszel test statistic of 3.46? Summary A large-sample approximation to Fisher’s Exact test is given by the MantelHaenszel (MH) chi-square statistic for simple analysis. The MH statistic is given by the formula:
2

(n 1)(ad bc) 2 n1n0 m1m0

The MH chi square statistic contains n-1 in the numerator whereas the largesample chi square version for comparing two proportions contains n in the numerator. In large-samples, either of these two chi square versions are essentially equivalent and will lead to the same conclusions about significance.

Large-Sample Confidence Interval for a Risk Ratio
We once again use the data from the mortality study on smoking behaviors of heart attack patients, this time to describe how to obtain a large-sample confidence interval for a risk ratio.

A risk ratio is a ratio of two proportions, each of which is a measure of cumulative incidence. If we were interested in the difference rather than the ratio between two cumulative incidences, the large sample confidence interval would be given by the commonly used confidence interval formula for two proportions shown here:

This formula says that we must add and subtract from the difference in the two estimated proportions 1.96 times the estimated standard error of this difference. The corresponding 95 percent confidence interval formula for a risk ratio is slightly more complicated. In contrast to the risk difference formula, the risk ratio formula looks like as shown to the right:

216 Chapter 12. Simple Analyses – 2x2 Tables are Not That Simple An equivalent version of the risk ratio formula, which helps explain where this formula comes from, is shown here: The confidence interval for a risk ratio is obtained by exponentiating a large sample confidence interval for the natural log of the risk ratio. The formula used for the estimated variance of the log of the risk ratio is actually an approximate, not an exact formula. There are two reasons why the formula for the risk ratio is more complicated than for the risk difference. First, the estimated difference in two proportions is approximately normally distributed, but the estimated ratio of two proportions is highly skewed. In contrast, the log of the risk ratio is more closely normally distributed. Second, the variance of a ratio of two proportions is complicated mathematically and is not equal to the ratio of the variances of each proportion. However, since the log of a ratio is the difference in logs, approximating the variance of a difference is much easier. We now apply the risk ratio formula to the mortality study data set. Substituting the values for the estimated risk ratio, the cumulative incidences in each group, and the sample sizes, we obtain the lower and upper confidence limits shown here:

Study Question (Q12.12) 1. Interpret the above results. Summary The confidence interval formula for a ratio of two proportions is more mathematically complicated than the formula for a difference in two proportions. The 95% risk ratio formula multiplies the estimated risk ratio by the exponential of plus or minus the quantity 1.96 times the square root of the variance of the log of the estimated risk ratio. This risk ratio formula is obtained by exponentiating a large sample confidence interval for the natural log of the risk ratio. The estimated ratio of two proportions is highly skewed whereas the log of the risk ratio is more closely normally distributed. The variance of the log of a risk ratio is mathematically easier to derive than is the variance of the risk ratio of itself, although an approximation is still required.

A Pocket Guide to Epidemiology 217 Quiz (12.13) For the cohort study data shown below, the estimated probability of death among continuing smokers (CS) is 0.303, and the estimated probability of death among smokers who quit (OS) is 0.256.

1. What is the estimated probability of death among the entire sample: ???. 2. Based on the computed probability values, use a calculator or computer to compute the value of the test statistic T for testing the null hypothesis that the RR equals 1. Your answer is T* = ???. Choices 0.279 0.475 0.491 0.533 0.721 2.20 2.23 2.27 2.31 2.36

3. Find the P-value for based on the computed T statistic (2.23) and assuming an upper-one-sided alternative hypothesis. Your answer is: P-value = ???. 4. Based on the P-value, you should reject the null hypothesis that RR=1 at the ??? significance level but not at the ??? significance level. Choices 0.003 0.013 0.021 0.120 1% 5%

The computed Z statistic for testing RR = 1 for these data is 2.23. 5. Using the computed Z statistic, what is the value of the Mantel-Haenszel chisquare statistic for these data? MH CHISQ = ???. 6. What is the P-value for a two-sided test of the null hypothesis that RR=1? (You may wish to use the chi square distribution tool located on lesson page 16-2 of the ActivEpi CD-ROM. ) P-value =???. 7. Fisher 's exact test is not necessary here because the sample size for this study is ???. Choices: 0.006 0.013 0.026 0.120 0.240 4.970 4.973 large small Calculating Sample Size for Clinical Trials and Cohort Studies
When a research proposal for testing an etiologic hypothesis is submitted for funding, it is typically required that the proposal demonstrate that the number of subjects to be studied is "large enough" to reject the null hypothesis if the null hypothesis is not true. This issue concerns the sample size advocated for the proposed study. The proposer typically will prepare a section of his/her proposal that describes the sample size calculations and resulting decisions about sample size requirements. Since most epidemiologic studies, even if considering a single dichotomous exposure variable, involve accounting for several (control) variables in the analysis, the methodology for determining sample size can be very complicated. As a result, many software programs have been developed to incorporate such multivariate complexities, e.g., Egret SIZ, PASS, and Power and Precision (a web-based package). The use of such programs is likely the

218 Chapter 12. Simple Analyses – 2x2 Tables are Not That Simple most mathematically rigorous and computationally accurate way to carry out the necessary sample size deliberations. Nevertheless, there are basic principles from which all sophisticated software derive, and such principles are conveniently portrayed in the context of a 2x2 table that considers the simple (i.e., crude) analysis of the primary exposure-disease relationship under study. Moreover, the use of sample size formulae for a simple analysis is often a convenient and non-black-box approach for providing a reasonable as well as understandable argument about sample size requirements for a given study. A description of such formulae now follows. All formulae for sample size requirements for hypothesis testing consider the two types of error that can be made from a statistical test of hypothesis. A Type I error occurs if the statistical test (incorrectly) rejects a true null hypothesis, and a Type II error occurs if the test (incorrectly) does not reject a false null hypothesis. The probability of making a Type I error is usually called , the significance level of the test. The probability of making a Type II error is usually called , and 1- is called the power of the test. All sample size formulae that concern hypothesis testing are aimed at determining that sample size for a given study that will achieve desired (small) values of and and that will detect a specific departure from the null hypothesis, often denoted as . Consequently, the investigator needs to specify values for , , and into an appropriate formula to determine the required sample size. For clinical trials and cohort studies, the sample size formula for detecting a risk ratio (RR) that differs from the null value of 1 by at least , i.e. ( = RR –1) is given by the formula:

n where (Z1

/2

Z1 ) 2 p q(r 1)
2

p 2 (RR 1) r
/2=

Z1 Z1

the 100(1 - /2) percent point of the N(0,1) distribution

= the 100(1 – ) percent point of the N(0,1) distribution

p2 = expected risk for unexposed subjects p = p2(RR + 1) / 2

q

1 p

r = ratio of unexposed to exposed subjects (Note: if the sample sizes are to be equal in the exposed and unexposed groups, then r = 1. When r does not equal 1, the above formula provides the sample size for the exposed group; to get the sample size for the unexposed group, use n x r.) To illustrate the calculation of n, suppose = .05, = .20, RR = 2, p2 = .04, and r = 3. Then: p = (.04)(2 + 1) / 2 = .06 and substituting these values into the formula for n yields:

n

(1.96 0.8416) 2 (.06)(.94)(3 1) 2 (.04)(2 1) 3

368.9

Thus, the sample size (n) needed to detect a risk ratio (RR) of 2 at an of .05 and a of .20, when the expected risk for exposed (p2) is .04 and the ratio of unexposed to exposed subjects (r) is 3, is 369 exposed subjects and 368.9 x 3 = 1,107 unexposed subjects. The above sample size formula can also be used to determine the sample size for estimating a prevalence ratio (i.e., PR) in a cross-sectional study; simply substitute PR for RR in the formula.

A Pocket Guide to Epidemiology 219

CASE-CONTROL STUDIES AND CROSSSECTIONAL STUDIES
Below we provide summaries of techniques for carrying out the analysis of a 2 x 2 table in case-control and cross-sectional studies. In case-control studies, the measure of effect of interest is the exposure odds ratio (i.e., EOR), and in crosssectional studies, the measure of effect of interest is either the prevalence odds ratio (i.e., POR) or the prevalence ratio (i.e., PR). Large sample Z tests and Mantel-Haenszel chi square tests for the EOR, POR, and PR use identical computational formula as used for tests about risk ratios (i.e., RR). Large sample confidence intervals for these parameters require different formulae for different parameters. Formulae for such tests and confidence intervals are provided at the end of this chapter. For more details on large sample tests for EOR, POR, and PR, as well as Fisher’s Exact Test for small samples and sample size calculation, see lesson page 12-6 in the ActivEpi CD ROM. Summary- Large Sample Tests When testing the hypothesis about an odds ratio (OR) in a case-control study, the null hypothesis can be equivalently stated as either EOR = 1 or p1 – p2 = 0, where p1 and p2 are estimated exposure probabilities for cases and non-cases. The alternative hypothesis can be stated in terms of the EOR either as EOR 1, EOR > 1, or EOR < 1, depending on whether the alternative is two-sided, upper one-sided, or lower one-sided, respectively. One version of the test statistic is a large-sample N(0,1) statistic used to compare two proportions. An alternative version is a large-sample chi square statistic, which is the square of the N(0,1) statistic. A Mantel-Haenszel large-sample chi square statistic can alternatively be used for the chi square test. Summary- Fishers Exact Test Fisher’s exact test provides a test of significance for an odds ratio as well as a risk ratio with the data are sparse. To use Fisher’s exact test, we must assume that the values on the margins of a 2x2 table are fixed values prior to the start of the study. To compute the P-value for Fisher’s exact test, we need to determine the probability distribution of the a cell frequency under the null hypothesis. Assuming fixed-marginals, the a cell has the hypergeometric distribution. Computer programs, including DataDesk for ActivEpi, are available to calculate the P-value for this test. Summary- Large Sample Confidence Intervals The formula for a 95% confidence interval for an odds ratio formula multiplies the estimated odds ratio by the exponential of plus or minus the quantity 1.96 times the square root of the variance of the log of the estimated odds ratio. The odds ratio formula is obtained by exponentiating a large sample confidence interval for the natural log of the odds ratio.

220 Chapter 12. Simple Analyses – 2x2 Tables are Not That Simple The variance of the log of an odds ratio is approximately equal to the sum of the inverses of the four cell frequencies in the 2x2 table layout.

COHORT STUDIES INVOLVING RATE RATIOS USING PERSON-TIME INFORMATION
In cohort studies that use person-time information, the measure of effect of interest is the rate ratio (i.e., incidence density ratio, IDR). Formulae for large sample tests and confidence intervals for the IDR are provided at the end of this chapter. For more details, see lesson page 12-7 in the ActivEpi CD ROM. Summary- Large Sample Tests When testing the hypothesis about a rate ratio in a person-time cohort study, the null hypothesis can be equivalently stated as either IDR = 1 or IR1 – IR0 = 0. IDR denotes the rate ratio (i.e., incidence density ratio) and IR1 and IR0 are the incidence rates for the exposed and unexposed groups. The alternative hypothesis can be stated in terms of the IDR 1, IDR > 1, or IDR < 1 depending on whether the alternative is two-sided, upper one-sided, or lower one-sided, respectively. One version of the test statistic is a large sample N(0,1) or Z statistic used to compare two incidence rates. An alternative version is a large-sample chi square statistic, which is the square of the Z statistic. Summary- Large Sample Confidence Interval The 95% CI formula for a rate ratio multiplies the estimate rate ratio by the exponential of plus or minus the quantity 1.96 times the square root of the variance of the log of the estimated rate ratio. This rate ratio formula is obtained by exponentiating a large sample confidence interval for the natural log of the rate ratio. The variance of the log of a rate ratio is approximately equal to the sum of the inverses of the exposed and unexposed cases.

Nomenclature
Table setup for cohort, case-control, and prevalence studies: Exposed Not Exposed Disease/cases a b No Disease/controls c d Total m1 m0 Table setup for cohort data with person-time information: Exposed Not Exposed Disease (New cases) I1 I0 Total disease-free PT1 PT0 person-time Total n1 n0 n

Total I PT

A Pocket Guide to Epidemiology 221
2 MH

Mantel-Haenszel chi square Cumulative incidence difference or risk difference, CI1 – CI0; same as risk difference (RD) Incidence density (or “rate”) in the population (I/PT) Incidence density (or “rate”) in the not exposed (I0/PT0) Incidence density (or “rate”) in the exposed (I1/PT1) Incidence density difference or rate difference, ID1 – ID0 Incidence density ratio or rate ratio: ID1 / ID0; same as Incidence Rate Ratio (IRR) Mantel-Haenszel

CID

ˆD I
ˆD I 0

ˆD I 1
IDD IDR MH

Formulae Statistical Tests
T statistic for the difference between two sample proportions

T

ˆ p1 ˆ ˆ p (1 p )

ˆ p0 1 n1 1 n0

where

ˆ p

ˆ ˆ n1 p1 n0 p0 n1 n0

Chi square statistic for a 2x2 table n(ad bc) 2 2

n1n0 m1m0
Mantel-Haenszel chi square statistic for a 2x2 table (n 1)(ad bc) 2 2 MH n1n0 m1m0 T statistic for the difference between two sample rates

T

I1

Ip0

where

Ip0 (1 p0 )
( I1 PT0 I 0 PT1 ) 2 IPT1 PT0

p0

PT1 PT

Chi square statistic for comparing two rates
2

222 Chapter 12. Simple Analyses – 2x2 Tables are Not That Simple

Confidence intervals
Large sample 95% confidence interval for the difference between two proportions (cumulative or risk difference)

ˆ p1

ˆ p0 1.96

ˆ ˆ p1 (1 p1 ) n1

ˆ ˆ p0 (1 p0 ) n0

Large sample 95% confidence interval for the risk ratio (ratio of two proportions)

ˆ RR exp

1.96

ˆ 1 CI 1 ˆ n CI
1 1

ˆ 1 CI 0 ˆ n CI
0 0

Large sample 95% confidence interval for the odds ratio

ˆ OR exp

1.96

1 a

1 b

1 c

1 d

Large sample 95% confidence interval for the incidence density ratio

ˆ IDR exp

1.96

1 I1

1 I0

Large sample 95% confidence interval for the incidence density difference

ˆ IDD 1.96

I1 PT12

I0 PT02

References
Basic Statistics References There are a great many beginning texts that cover the fundamental concepts and methods of statistics and/or biostatistics. (Note: biostatistics concerns applications of statistics to the biological and health sciences.) Consequently, it is not our intention here to provide an exhaustive list of all such texts, but rather to suggest a few references that this author has used and/or recommends. Among these, we first suggest that you consider Velleman 's ActivStats CD ROM text, which has the same format has ActivEpi. Suggested references include:

Kleinbaum DG, Kupper LL, Muller KA, Nizam A Applied Regression Analysis and Other Multivariable Methods, 3rd Edition. Duxbury Press, 1998. (Chapter 3 provides a compact review of basic statistics concepts and methods.) Moore D. The Active Practice of Statistics. WH Freeman Publishers, 1997 (This book is designed specifically to go with ActivStats and matches it closely.) Remington RD and Schork MA. Statistics with Applications to the Biological and Health Sciences. Prentice Hall Publishers, 1970. Velleman P. ActivStats - A Multimedia Statistics Resource (CDROM), Addison-Wesley Publishers, 1998. Weiss N. Introductory Statistics, 6th Edition. Addison-Wesley Publishing Company, Boston, 2002.

A Pocket Guide to Epidemiology 223 References on Simple Analysis
Fleiss JL. Confidence Intervals for the odds ratio in case-control studies: the state of the art. J Chronic Dis 1979;32(1-2):69-77. Goodman SN. Toward evidence-based medical statistics. 1: The P value fallacy. Ann Intern Med 1999;130(12):995-1004. Goodman SN. Toward evidence-based medical statistics. 2: The Bayes factor. Ann Intern Med 1999;130(2):1005-13. Kleinbaum DG, Kupper LL, Morgenstern H. Epidemiologic Research: Principles and Quantitative Methods, Chapter 15 , John Wiley and Sons, 1982. Miettinen O. Estimability and estimation in case-referent studies. Am J Epidemiol 1976;103(2):226-235. Mantel N, Haenszel W. Statistical aspects of the analysis of data from retrospective studies of disease. J Natl Cancer Inst 1959;22(4):719-48. Thomas DG. Exact confidence limits for an odds ratio in a 2x2 table. Appl Stat 1971;20:105-10.

Answers to Study Questions and Quizzes
Q12.1 1. RR, the ratio or two proportions, or OR, the ratio of two odds, each of the form p/(1-p). 2. The usual null hypothesis for a risk ratio is RR = 1, where 1 is the null value of the risk ratio. 3. The usual null hypothesis for an odds ratio is OR = 1, where 1 is the null value of the odds ratio. Q12.2 1. The pooled estimate of the common proportion is given by {(.7972 x 1667) + (.2565 x 425)} / {1667 + 425} = .6874. Q12.3 1. The 2-sided alternative A1, in the lower right corner. 2. Because the normal curve is symmetric, the left and right tails have equal areas. Thus, compute one tail’s area and then multiply by two. 3. For an upper one-sided alternative, the P-value is the area under the normal curve to the right of T*=12.0. From the table, we find P .01: Do not reject H0. b) P = .023: Reject H0. 8. c) P < .001: Reject H0. d) P = .54: Do not reject H0. e) P = .0002: Reject H0. 9. a) P > .01: Do not reject H0. b) P = .023: Do no reject H0. 10. c) P < .001: Reject H0. d) .01 < P < .05: Do not reject H0. e) P = .0002: Reject H0. Q12.4 1. hypothesis testing 2. interval estimation 3. hypothesis testing 4. interval estimation 5. significance level 6. alternative hypothesis 7. rejection region, null hypothesis, statistically 8. P-value, null hypothesis 9. not reject

224 Chapter 12. Simple Analyses – 2x2 Tables are Not That Simple
10. .01, .001 Q12.5 1. The standard error used here does not assume that the null hypothesis is true. Thus, the variance for each proportion must be computed separately using its sample proportion value. Q12.6 1. 100% confidence. 2. The 99% confidence interval would be the widest and the 90% confidence interval would be the narrowest. 3. Dataset A gives the more precise estimate because it’s confidence interval is narrower than that for Dataset B. Q12.7 1. estimated standard error 2. difference 3. population parameter 4. upper limit 5. 95% 6. 5% Q12.8 1. Yes, because 7.04 is larger than 6.635, which is the .99 percent point of the chi square distribution with 1 df. 2. No, because 7.04 is less than 7.879, which is the .995 percent point of the chi square distribution with 1 df. 3. Yes, because the P-value for a twosided test is the area above 7.04, which is less than the area above 6.635, which is .01. 4. No, because the P-value is the area above 7.04, which is greater than the area above 7.879, which is .005. 5. Yes, because the upper one-sided test using the normal curve was significant at the 1 percent level. 6. Yes, the one-tailed P-value using the normal distribution was .0040, which is less than .005. Q12.9 1. The correct P-value is the area under the normal curve below +2.65, which is 1 minus the area above 2.65, which is calculated to be 1 - .004 = .996. The value of .996 can equivalently be obtained from the chi square curve by taking one half of .008 and subtracting from 1, i.e., 1 – (.008 / 2) = .996. It would be incorrect, therefore, to take one-half of the area of .008 under the chi-square curve nor would it be correct to take one-half of 1 minus .008. 2. Do not reject the null hypothesis because the P-value is very high. 3. The correct P-value is the area under the normal curve below –2.65, which is .004. This can also be obtained by taking half of the area above the chi square value of 7.04, which is .008/2 = .004. 4. Reject the null hypothesis because the P-value is very small, and much smaller than .05 and .01. Q12.10 1. b = m1 – a; c = n1 – a; d = m0 – n1 + a 2. b = 2; c = 1; d = 6 3. b = 1; c = 0; d = 7 4. RR = (5/5) / (1/8) = 8 5. Yes 6. No. An a cell value greater than 5 is not possible because the assumed fixed column marginal of 5 would then be exceeded. 7. P(a=4 or 5|RR=1) = P(a=4|RR=1) + P(a=5|RR=1). This tells us how rare our observed results are under the null hypothesis. It is the P-value for testing this hypothesis. 8. Since P is greater than .05, we fail to reject at the .05 significance level for the null hypothesis that the RR = 1. Q12.11 1. The one-sided p-value is .0630 / 2 = .0315 2. The large-sample assumption does not hold. 3. The sample size n = 13 is not large, so that the difference between n = 13 and n – 1 = 12 has a stronger effect on the calculation of each test statistic. Q12.12 1. This confidence interval has a 9% probability of covering the true risk ratio that compares continuing smokers to smokers who quit. Even though the confidence interval contains the null value of 1, it is wide enough to suggest that the true risk ratio might be either close to one or as large as 3.6. In other words, the point estimate of 2.1 is somewhat imprecise, and the true effect

A Pocket Guide to Epidemiology 225 of quitting smoking after a heart attack may be either very weak or very strong. Q12.13 1. 520/1863 = 0.279 2. 2.23: T* = (.303 - .256) / [sqrt{.279 * (1-.279) * [(1/915) + (1/948)]}] = 2.23 3. .013 4. 5%, 1% 5. 4.970: MH chi square = (1862 * 2.232)/1863 = 4.970 6. 0.026 7. large

CHAPTER

13

CONTROL- WHAT IT’S ALL ABOUT
In previous chapters, we have discussed and illustrated several important concepts concerning the control of additional (extraneous) variables when assessing a relationship between an exposure variable and a health-outcome variable. In this chapter, we briefly review these concepts and then provide an overview of several options for the process of control that are available at both the design and analysis stages of a study.

What do we Mean by Control?
Suppose we are studying whether there is a link between exposure to a toxic chemical and the development of lung cancer in a chemical industry. To answer this question properly, we would want to isolate the effect of the chemical from the possible influence of other variables, particularly age and smoking status, two known risk factors for lung cancer. That is, our goal is to determine whether or not exposure to the chemical contributes anything over and above the effects of age and smoking to the development of lung cancer. Variables such as age and smoking in this example are often referred to as control variables. When we assess the influence of such control variables on the E D relationship, we say we are controlling for extraneous variables. By extraneous, we simply mean that we are considering variables other than E and D that are not of primary interest but nevertheless could influence our conclusions about the E D relationship. In general, we typically carry out a simple analysis of an exposure-disease relationship as the starting point for more complicated analyses that we will likely have to undertake. A simple analysis allows us to see the crude association between exposure and disease and therefore allows us to make some preliminary insights about the exposure-disease relationship. Unfortunately, a simple analysis by definition ignores the influence that variables other than the exposure may have on the disease. If there are other variables already known to predict the disease, then the conclusions suggested by a simple analysis may have to be altered when such risk factors are taken into account. Consequently, when we control for extraneous variables, we assess the effect of the exposure E on the disease D at different combinations of values of the variables we are controlling. When appropriate, we evaluate the overall E D relationship by combining the information over the various combinations of control values.

228 Chapter 13. Control – What It’s All About

Study Questions (Q13.1) Consider a case-control study to assess whether a certain toxic chemical (E) is associated with the development of lung cancer (D) in a chemical industry. Suppose we wish to consider the control of age and smoking status. Assume that we categorize age into three groups: below 40, 40-55, and over 55. We also categorize smoking as “ever smoked” versus “never smoked”. 1. How many combinations are there of the categories of age and smoking? Two kinds of pooled analyses with these data are: Pool 2x2 tables of these combinations into one overall “pooled” table and compute an odds ratio for this pooled table, and Compute an odds ratio for each 2x2 table corresponding to each combination and then average these separate odds ratios in some way. 2. Which of these two analyses controls for age and smoking? Several questions arise when considering the control of extraneous variables. Why do we want to control in the first place? That is, what do we accomplish by control? What are the different options that are available for carrying out control? Which option for control should we choose in our study? Which of the variables being considered should actually be controlled? What should we do if we have so many variables to control that we run out of data? These questions will be considered in the activities to follow. Summary When assessing an E D relationship, we determine whether E contributes anything over and above the effects of other known predictors (i.e., control variables) of D. When we assess the influence of control variables, we say we are controlling for extraneous variables. A simple analysis ignores the control of extraneous variables.

A Pocket Guide to Epidemiology 229 Controlling assesses the E D relationship at combinations of values of the control variables. When appropriate, controlling assesses the overall E D relationship after taking into account control variables.

Reasons for Control
The typical epidemiologic research question assesses the relationship between one or more health outcome variables, D, and one or more exposure variables, E, taking into account the effects of other variables, C, already known to predict the outcome. When there is only one D and one E, and there are several control variables, the typical research question can be expressed as shown here, where the arrow indicates that the variable E and the C variables on the left are to be evaluated as predictors of the outcome D, on the right. Why are the C variables here? That is, what are the reasons why we want to control for the C 's? One reason for control is to ensure that whatever effect we may find of the exposure variable cannot be explained away by variables already known to have an effect on the health outcome. In other words, we want to make sure we have accounted for the possible confounding of the E D relationship due to the influence of known risk factors for the health outcome. A second reason for control is to ensure that we remove any variability in the estimate of the E D effect contributed by other known predictors. We might gain precision in our effect estimate, for example, a narrow confidence interval, as a result of controlling. In some situations there may be a loss of precision when controlling for confounders. A third reason for control is to allow us to assess whether the effect of the exposure may vary depending on the characteristics of other predictors. For example, there may be a strong effect of exposure for smokers but no effect of exposures for non-smokers. This issue concerns interaction, or effect modification. These are the primary three reasons for controlling: 1) to control for confounding; 2) to increase precision; and 3) to account for the possibility of effect modification. All three reasons are important, but there is nevertheless an ordering of when they should be considered in the course of an analysis. The possibility of effect modification should be considered first, because if there is an effect modification, then a single adjusted estimate that controls for confounding may mask the fact that the E D relationship differs for different categories of a control variable.

230 Chapter 13. Control – What It’s All About Once effect modification is addressed or found to be absent, confounding should be considered, particularly in terms of those control variables not found to be effect modifiers. Confounding should be assessed prior to precision because confounding concerns the validity of an estimate. Precision only concerns random error. We would rather have a valid estimate than a narrow confidence interval around a biased estimate. Study Questions (Q13.2) Consider again a case-control study to assess whether a certain toxic chemical (E) is associated with the development of lung cancer (D) in a chemical industry, where we wish to control for age and smoking status. Also, assume that we categorize age into three groups: below 40, 40-55, and >55 years of age; and we categorize smoking as “ever smoked” versus “never smoked.” 1. True of False. We can assess confounding of either age or smoking by determining whether the E D relationship differs within different categories of either age or smoking or both combined. 2. True or False. A more precise estimate of the odds ratio (OR) for the E D association will be obtained if we control for both age and smoking status. Suppose that when controlling for both age and smoking status, the OR for the E D association is 3.5, but when ignoring both age and smoking status, the corresponding crude OR is 1.3. 3. Does this indicate confounding, precision, or interaction? Suppose that when controlling for age and smoking status, the adjusted OR is 3.5, as above, with a 95% confidence interval ranging from 2.7 to 4.5, but that the crude OR of 1.3 has a 95% confidence interval from 1.1 to 1.5. 4. Which of these to OR’s is more appropriate? Suppose in addition to the above information, you learned that the estimated OR relating E to D is 5.7 for smokers but only 1.4 for non-smokers? 5. Would you want to control for confounding of both age and smoking? Summary The typical epi research question assesses the relationship of one or more E variables to one or more D variables controlling for several C variables. The three reasons to control are confounding, precision, and effect modification. The possibility of effect modification should be considered first, followed by confounding, and then precision.

A Pocket Guide to Epidemiology 231

Options for Control
Design Options
Suppose you wish to assess the possible association of personality type and coronary heart disease. You decide to carry out a cohort study to compare the CHD risk for a group of subjects with Type A personality pattern with the corresponding risk for Type B subjects. You plan to follow both groups for the same duration, say 5 years. Your exposure variable, E, is therefore dichotomous. You recognize that age, gender, ethnicity, blood pressure, smoking status, and cholesterol level are important CHD risk factors that you need to observe or measure for control in your study. You also recognize that there are other factors such as genetic factors, daily stress level, physical activity level, social class, religious beliefs, that you might also like to consider but you don 't have the resources to measure.

How do you carry out your study to control for any or all of the variables we have just mentioned? That is, what are your options for control? Some of your options need to be carried out at the study design stage prior to data collection. Other options are carried out during the analysis stage after the data has been obtained. It is possible to choose more than one option in the same study. We first focus on the design options. One design option is randomization. You might wish to randomly assign an initial disease-free individual to either Type A or Type B personality type. If you could randomize, then the variables you want to control for, even those you don 't actually measure, might be distributed similarly for both exposure groups. However, this option is unavailable here. You can 't force people to be one personality type or another; they are what they are. A second design option is called restriction. This means specifying a narrow range of values for one or more of the control variables. For example, you might decide to restrict the study to African-Americans, to women, to persons older than 50, or to all three, but not to restrict any of the other variables on your list.

232 Chapter 13. Control – What It’s All About Study Questions (Q13.3) 1. What is a drawback to limiting the study to women only? 2. Other than generalizing to other age groups, what is another drawback to limiting the age range to persons over 50? A third design option is matching. Matching imposes a “partial restriction” on the control variable being matched. For example, if we match on smoking status using what is called pair matching, our cohort would consist of pairs of subjects, where each pair would have a Type A subject and Type B subject who are either both smokers or both non-smokers. Smoking status would therefore not be restricted to either all smokers or all non-smokers. What would be restricted, however, is the smoking status distribution, which would be the same for both Type A and Type B groups. Matching is often not practical in cohort studies such as the study described here, so it is rarely used in cohort studies. Rather it is most often used in case-control studies or in clinical trials. Study Questions (Q13.3) continued Suppose you carry out a case-control study to compare CHD risks for Type A with Type B subjects. You decide to pairmatch on both smoking status (ever versus never) and gender. Your cases are CHD patients identified from a cardiovascular disease registry and your controls are disease-free non-CHD subjects that are community-based. 3. What is the smoking status and gender of a control subject who is matched with a male non-smoking case? 4. If there are 110 cases in the study, what is the total number of study subjects? (Assume pair matching, as described above.) 5. Is there any restriction on the smoking status or gender of the cases? 6. Is there any restriction on the smoking status or gender of the controls? Summary Three design options for controlling extraneous variables are: Randomization – randomly allocating subjects to comparison groups Restriction – specifying a narrow range of possible values of a control variable. Matching – a partial restriction of the distribution of the comparison group.

Analysis Options
What control options are available at the analysis stage of a study? We 'll continue the illustration of a 5-year cohort study to assess the possible association of personality type and coronary heart disease. Once the data are collected, the most direct and logical analysis option is a stratified analysis. In our cohort study, if we have not used either restriction or matching at the design stage, stratification can be done by categorizing all control variables and forming combinations of categories called strata. For example, we might categorize age in three groups, say under 50, 50-60, and 60 and over; ethnicity into non-whites verses whites; diastolic blood pressure into below 95 and 95 or higher; and HDL level at or below 35 versus above 35. Because there are 6 variables being categorized into 3 categories for age and 2 categories for the other 5

A Pocket Guide to Epidemiology 233 variables, the total number of category combinations is 3 times 25, or 96 strata.

An example of a stratum is subjects over 60, female, non-white, with a diastolic blood pressure greater than 95, a smoker, and with HDL level below 35. For each stratum we form the 2x2 table that relates exposure, here personality type, to the disease, here, CHD status. A stratified analysis is then carried out by making decisions about the E-D relationship for individual strata and if appropriate, combining the information over all strata to provide an overall adjusted estimate that controls for all variables together.

Study Questions (Q13.4) 1. When would it not be appropriate to compute an overall adjusted estimate that combines the information over all strata? 2. How would you assess the E D relationship within any given stratum? 3. For the cohort study illustrated here, what is the biggest obstacle in carrying out stratum-specific analyses? 4. How might you carry out stratified analyses that avoid dealing with a large number of strata containing zero cells? 5. What would be an advantage and a disadvantage of doing several stratified analyses one variable at a time? A major problem with doing stratified analysis when there are many variables to control is that you quickly run out of subjects. An alternative option that gets around this problem is to use a mathematical model. A mathematical model is a mathematical expression or formula that describes how an outcome variable, like CHD status in our example, can be predicted from other variables, which in our example are the exposure variable and the control variables we have measured or observed. In other words, we have a variable to be predicted, often referred to as the dependent variable and typically denoted Y, and predictors, often called

234 Chapter 13. Control – What It’s All About independent variables and typically denoted with X 's.

When modeling is used, we do not have to split up the data into strata. Instead, we obtain a formula to predict the dependent variable from the independent variables. We can also use the formula to obtain estimates of effect measures such as risk ratios or odds ratios. But modeling has difficulties of its own. These include the choice of the model form to use, the variables to be included in the initial and final model, and the assumptions required for making statistical inferences. For dichotomous dependent variables like CHD in our example, the most popular mathematical model is called the logistic model. Study Questions (Q13.4) continued Suppose we want to use mathematical modeling to assess the relationship between personality type (E) and CHD status (D) controlling for age, gender, ethnicity, diastolic blood pressure (DBP), smoking (SMK), and high-density lipoprotein (HDL). 6. True or False. The only possible choices for the independent variables in this model are E and the above 6 control variables. 7. True or False. In a mathematical model, continuous variables must be categorized. 8. True or False. In a case-control study, the dependent variable is exposure status. Suppose f(X1, X2, X3, X4, X5, X6, X7) represents a mathematical formula that provides good prediction of CHD status, where X1 through X7 denote E and the 6 control variables described above. 9. True or False. If we substitute a person’s specific values for X1 through X7 into the formula, we will determine that person’s correct CHD status. Summary At the analysis stage, there are two options for control: Stratified analysis - categorize the control variables and form combinations of categories or strata. Mathematical modeling – use a mathematical expression for predicting the outcome from the exposure and the variables being controlled. Stratified analysis has the drawback of running out of numbers when the number of strata is large. Mathematical modeling has its drawbacks, including the choice of model and the variables to be included in the initial and final model.

A Pocket Guide to Epidemiology 235 Quiz (Q13.5) The three primary reasons for controlling in an epidemiological study, listed in the order that they should be assessed, are: 1. ??? 2. ??? 3. ??? Choices: Confounding Effect Modification Matching Stratification Mathematical Modeling Precision Randomization Restriction There are three options for control that can be implemented at the design stage. 4. ??? is a technique for balancing how unmeasured variables are distributed among exposure groups. 5. ??? limits the subjects in the study to a narrow range or values for one or more of the control variables. 6. In a case-control study, ??? ensures that the some or possibly all control variables have the same or similar distribution among case and control groups. Choices Matching Mathematical Modeling Optimization Randomization Restriction Stratification 7. Stratification is an analysis technique that starts by dividing the subjects into different ??? based on categories of the ??? variables. 8. A major problem with stratified analyses is having too many ??? variables, which can result in ??? data in some strata. Choices control diseaseexposure sparse strata treatment large numbers random samples

9. In mathematical modeling we use a formula to predict a ??? variable from one or more ??? variables. 10. The problems with using a mathematical model include the choice of the ??? of the model, deciding what 111 to include in the model, and the ??? required for making statistical inferences from the model. Choices assumptions cases complexity control dependent independent subjects treatments variables form

Randomization
Randomization allocates subjects to exposure groups at random. In epidemiologic research, randomization is used only in experimental studies such as clinical or community trials, and is never used in observational studies. What does randomization have to do with the control of extraneous variables? The goal of randomization is comparability. Randomization tends to make the comparison groups similar on demographic, behavioral, genetic, and other characteristics except for exposure status. The investigator hopes, therefore, that if the study finds any difference in health outcome between the comparison groups, that difference can only be attributable to their difference in exposure status.

236 Chapter 13. Control – What It’s All About

For example, if subjects are randomly allocated to either a new drug or a standard drug for the treatment of hypertension, then it is hoped that other factors, such as age and sex, might have approximately the same distribution for subjects receiving the new drug as for subjects receiving the standard drug. Actually, there is no guarantee even with randomization that the distribution of age, for example, will be the same for the two treatment groups. The investigator can always check the data to see what has happened regarding any such characteristic, providing the characteristic is measured or observed in the study. If, for example, the age distribution is found to be different between the two treatment groups, the investigator can take this into account in the analysis by stratifying on age. An important advantage of randomization is what it offers for those variables not measured in the study. Variables that are not measured obviously cannot be taken into account in the analysis. Randomization offers insurance, though no guarantee, that such unmeasured variables are similarly distributed among the different exposure groups. In observational studies, on the other hand, the investigator can account for only those variables that are measured, allowing more possibility for spurious conclusions because of unknown effects of important unmeasured variables. Study Questions (Q13.6) Suppose you plan to do a case-control study to assess whether personality type is a risk factor for colon cancer. 1. Can you randomly assign your study subjects to different exposure groups? Suppose you plan a clinical trial to compare two anti-hypertensive drugs. You wish to control for age, race, and gender, but you also wish to account for possible genetic factors that you cannot measure. 2. Can you control for specific genetic factors in your analysis? 3. Will the two drug groups have the same distributions of age, race, and gender? 4. What do you hope randomization will accomplish regarding the genetic factors you have not measured? Summary Experimental studies use randomization whereas observational studies do not use randomization. The goal or randomization is comparability. Randomization tends to make comparison groups similar on other factors to be controlled. An important advantage of randomization is that it tends to make variable not measured similarly distributed among comparison groups.

A Pocket Guide to Epidemiology 237 There is not guarantee that randomization will automatically make comparison groups similar on other factors.

Restriction
Restriction is another design option for control in which the eligibility of potential study subjects is narrowed by restricting the categories of one or more control variables. Restriction can be applied to both continuous and categorical variables. For a categorical variable, like gender, restriction simply means that the study is limited to one or more of the categories. For a continuous variable, restriction requires limiting the range of values, such as using a narrow age range, say from 40 to 50 years of age. Restriction typically provides complete control of a variable. It is convenient, inexpensive, and it requires a simple analysis to achieve control. For example, if a study is restricted to females only, the analysis does not require obtaining an adjusted effect that averages over both genders. The main disadvantage of restriction is that we cannot generalize our findings beyond the restricted category. For continuous variables, another disadvantage is that the range of values being restricted may not be sufficiently narrow, so there may still be confounding to be controlled within the chosen range.

Given a list of several control variables to measure, we typically use restriction on a small number of variables. This allows hypotheses to be assessed over several categories of most control variables, thereby allowing for more generalizability of the findings. For example, if we want to control for age, gender, ethnicity, diastolic blood pressure, smoking, and HDL level, we would likely use restriction for no more than two or three of these variables, say age and gender. Restriction may be used at the analysis stage even if not used at the design stage. For example, even though the study sample may include several ethnic groups, we may decide to only analyze the data for one of these groups, particularly if other ethnic groups have relatively few subjects. However, it 's more advantageous to choose restriction at the design stage to gain precision in the estimated effect or to reduce study costs. For example, for a fixed study size or fixed study cost, restricting ethnicity to African-Americans at the design stage will provide more African-American subjects, and therefore more precision in effect

238 Chapter 13. Control – What It’s All About measures for African-Americans, than would be obtained if the design allowed several ethnic groups to be eligible. Summary Restriction is a design option that narrows the eligibility of potential study subjects by restricting the categories of one or more control variables. Restriction can be applied to both categorical and continuous variables. Restriction typically provides complete control, is convenient, inexpensive, and requires a simple analysis to achieve control. The main disadvantage of restriction is not being able to generalize findings beyond the restricted category. For continuous variables, another disadvantage is the possibility of residual confounding within the range of restricted values. Restriction may be used in the analysis stage, but if used at the design stage, precision may be gained and/or study costs may be reduced. Quiz (Q13.7) Label each of the following statements as True or False. 1. Restriction can only be used with categorical variables. ??? 2. An advantage of restriction is that it requires only a simple analysis to achieve control. ??? 3. One disadvantage of restriction is that it can be expensive to administer. ??? 4. Restriction can be used at both the design and analysis stage. ???

Matching
Matching is a design option that can be used in experimental studies and in observational cohort studies, but is most widely used in case-control studies. A general definition of matching that allows other designs is given in Chaptersson 15. There are generally two types of matching: individual matching and frequency matching. When individual matching is used in a case-control study, one or more controls are chosen for each case so that the controls have the same or similar characteristics on each of the variables involved in the matching. For example, if we match on age, race, and sex, and a given case is, say, 40 years old, black, and male, then the one or more controls matched to this case must also be close to or exactly 40 years old, black, and male. For continuous variables, like age, the categories used for matching must be specified prior to the matching process. For age, say, if the matching categories are specified as 10-year age bands that include the age range 35-45, then the control match for a 40-year-old case must come from the 35-45 year old age range. Here, we are restricting the distribution of age, race, and gender in the control group to be the same as in the case group. But we are not restricting either the values or the distribution of age, race, and sex for the cases. That 's why we say that matching imposes a partial restriction on the control variables being matched.

A Pocket Guide to Epidemiology 239

If frequency matching is used in a case-control study, then the matching is done on a group rather than individual basis. For example, suppose we wish to frequency match on race and gender in a case-control study, where the cases have the race-by-gender breakdown shown here:

The controls then must be chosen as a group to have the same distribution as the cases over the four races by gender strata. If we want to have twice as many controls as cases, then the race by gender breakdown for controls will follow the same distribution pattern as the cases. Several issues need to be considered when matching. First, what are the advantages and disadvantages of matching? A major reason for matching is to gain efficiency or precision in the estimate of effects, say, the odds ratio. Should we match at all, or should we choose the cases and controls without matching? There is no simple answer to this question, but a rough guideline is to only match on variables that you think will be strong confounders in your data. How many controls should we choose for each case? A rough guideline here is that usually no more than four controls per case will be necessary in order to gain precision. How do we analyze matched data? The answer here depends in part on whether or not there are other variables to be controlled in the analysis besides the matching variables. In particular, if the only variables being controlled are involved in the matching, than the appropriate analysis is a special kind of stratified analysis. But if in addition to the matching variables, there are other variables to be controlled, then the appropriate analysis involves mathematical modeling, usually using logistic regression methods.

240 Chapter 13. Control – What It’s All About

Summary Matching can be used in both experimental and observational studies, and is most often used in case-control studies. There are two types of matching: individual matching and frequency matching. A major reason for matching is to gain efficiency or precision in the estimate of effect. Usually no more than four controls per case will be necessary in order to gain precision. If the only variables being controlled are involved in the matching, then use stratified analysis. If there are other variables to be controlled, then use mathematical modeling. Quiz (Q13.8) Label each of the following statements as True or False. 1. Matching is used mostly with observational cohort studies. ??? 2. For continuous variables, the ranges used for creating matching categories must be specified prior to the matching process. ??? 3. For frequency matching, there can be more controls than cases. ??? 4. Stratified Analysis is typically used for matched data to control for variables other than those involved in the matching. ???

Stratified Analysis
Stratified analysis is an analysis option for control that involves categorizing all study variables, and forming combinations of categories called strata. If both the exposure and the disease variables are dichotomous, then the strata are in the form of several two by two tables. The number of strata will depend on how many variables are to be controlled and how many categories are defined for each variable.

Study Questions (Q13.9) 1. If three variables are to be controlled and each variable is dichotomized, how many strata are obtained?

A Pocket Guide to Epidemiology 241 2. If the control variables are age, race, and gender, give an example of one of the strata. Once the strata are defined, a stratified analysis is carried out by making stratum-specific simple analyses and, if appropriate, by making an overall summary assessment of the E D relationship that accounts for all control variables simultaneously. Both the stratum-specific analyses and the overall summary analyses will typically involve computing and interpreting a point estimate of the effect, say a risk ratio, a confidence interval for the point estimate, and a test of hypothesis for the significance of the point estimate. For an overall summary assessment, the point estimate is an adjusted estimate that is some form of weighted average of stratum-specific estimates. The confidence interval is an interval estimate around this weighted average, and the test of hypothesis is a generalization of the Mantel-Haenszel chi-square that now considers several strata. Summary Stratified analysis involves the following steps: Categorize all variables Form combinations of categories (i.e., strata) Carry out stratum-specific analyses Carry out an overall E D assessment, if appropriate Both stratum-specific analyses and overall assessment require point and interval estimates, and a test of hypothesis. For overall assessment: The point estimate is an adjusted estimate that is a weighted average of stratum-specific estimates. The confidence interval is an interval estimate around the adjusted (weighted) estimate. The test of hypothesis is a generalization of the Mantel-Haenszel chi square test. Quiz (Q13.10) Label each of the following statements as True or False. 1. Stratified analysis is an analysis option for control that involves categorizing all study variables, and forming combinations of categories called strata. ??? 2. Tests of hypothesis are not appropriate for stratified analyses. ??? 3. When carrying out stratum-specific analyses, the point estimate is typically computed as a weighted average. ??? 4. When carrying out stratum-specific analyses, an appropriate test statistic for large samples is a Mantel-Haenszel chi square statistic. ??? 5. If it is appropriate to carry out overall assessment over all strata, a recommended test statistic for large samples is a Mantel-Haenszel chi square statistic. . . . ??? 6. When carrying out overall assessment over all strata, a Mantel-Haenszel chi square statistic is always appropriate for large samples. ???

242 Chapter 13. Control – What It’s All About

Mathematical Modeling
See lesson page 13-3 in the ActivEpi CD ROM for an introduction to mathematical modeling, including the terms independent and dependent variables, and expected value of a model. Also, a brief introduction to the logistic model is also provided.

References
Feinstein AR. Clinical Biostatistics. Mosby Publishers, St. Louis, 1977. Hoes AW, Grobbee DE, Valkenburg HA, Lubsen J, Hofman A. Cardiovascular risk and allcause mortality: a 12 year follow-up study in the Netherlands. Eur J EpidemioI 1993;9(3):285-92. Kleinbaum DG, Kupper LL, Morgenstern H. Epidemiologic Research: Principles and Quantitative Methods. John Wiley and Sons Publishers, New York, 1982. Kleinbaum DG, Klein M. Logistic Regression: A Self-Learning Text, 2nd Ed. Springer Verlag Publishers, 2002. Miettinen OS. Matching and design efficiency in retrospective studies. Am J Epidemiol 1970;91(2):111-8.

Answers to Study Questions and Quizzes
Q13.1 1. 3 x 2 = 6 combinations 2. Approach 2 controls for age and smoking, since it considers what happens when variables are controlled. Approach 1 ignores control of age and smoking. Q13.2 1. False. The statement addresses the question of interaction/ effect modification, not confounding. 2. False. The precision obtained will depend on the data; there is no guarantee that precision is always gained by controlling for extraneous variables. 3. Confounding, since the statement concerns what happens when we compare a crude estimate of effect with an adjusted estimate of effect. 4. If age and smoking are risk factors for lung cancer (they are), then the OR of 3.5 is more appropriate because it controls for confounding, even though it is less precise than the crude estimate; i.e., validity is more important than precision. 5. No, estimated OR’s of 5.7 for smokers and 1.4 for non-smokers indicate strong interaction due to smoking (provided the observed interaction is statistically significant). An assessment of confounding would require comparing a crude estimate to an adjusted estimate, but use of the latter would not be appropriate because it would mask the presence of strong interaction. Q13.3 1. You cannot generalize your results to men; that is, generalizing to men is an “external validity” problem. 2. The age group of persons over 50 is not necessarily narrow enough to completely control for age. In particular, there may still be “residual” confounding due to age within the age group over 50. 3. The control will have the same smoking status and gender as the case, i.e., the control will be a non-smoking male. 4. 220, since there will be 110 cases and 110 matched controls. 5. No, cases can be either male or female and either smokers or non-smokers, and they can have any distribution possible of each of these variables. 6. Yes, the controls are restricted to have the same distribution of both smoking status and gender as the cases. Q13.4 1. It would not be appropriate to compute an overall adjusted estimate of there is strong evidence of interaction, e.g., if the estimated risk ratios in two or more strata are both statistically and meaningfully different. 2. Carry out a simple analysis for the given stratum by computing a point estimate of effect (e.g., a risk ratio), a confidence interval around the point estimate, and a test of hypothesis about the significance of this estimate. 3. There are 96 strata in all, so that it is highly likely that the entire dataset will be greatly thinned out upon stratification, including many strata containing one or more zero cells. 4. Do a stratified analysis one variable or two variables at a time, rather than all the variables being controlled simultaneously.

A Pocket Guide to Epidemiology 243
An advantage is that you can make some preliminary insights about confounding and interaction for every control variable. A disadvantage is that you will not be controlling for all variables simultaneously. 6. False. If age is continuous, then age2 might also be used. Similarly for other continuous variables. Also, product terms like E x age, age x gender might be used. 7. False. Continuous variables can be treated as either continuous or categorical, depending on the investigator’s judgment. However, for a stratified analysis, continuous variables must be categorical. 8. False. Even though cases and controls are selected first and previous exposure then determined, case-control status is the dependent variable in a mathematical model because it represents the health outcome variable being predicted. 9. False. Mathematical models rarely, if ever, perfectly predict the outcome variable. There is always some amount of error that represents the difference between a predicted value and the observed value. Q13.5 1. Effect modification 2. Confounding 3. Precision 4. Randomization 5. Restriction 6. Matching 7. Strata, Control 8. Control, Sparse 9. Dependent, Independent 10. Form, Variables, Assumptions Q13.6 1. No. In a case-control study, exposure status is determined only after cases and controls are selected. Therefore, randomization to exposure groups (i.e., personality types) is not possible in case-control studies. 2. No. You cannot control for factors in your analysis that you have not measured. 3. Not necessarily. Randomization would tend to make the distribution of age, race, and gender similar in the two drug groups, but there is no guarantee that they will be the same. 4. You hope that the distribution of genetic factors is similarly distributed within the two drug groups, even though these factors have not been measured. Moreover, you hope that, for any other unmeasured factors, randomization will distribute such factors similarly over the groups being considered. Q13.7 1. False – Restriction can also be used to limit the range of values of a continuous variable. 5. 2. True – An advantage of restriction is that it is inexpensive to administer. 3. False 4. True Q13.8 1. False – Matching is used mostly with casecontrol studies. 2. True 3. True 4. False – If in addition to the matching variables, there are other variables to be controlled, then the appropriate analysis involves mathematical modeling using logistic regression methods. Q13.9 1. 2 x 2 x 2 = 8 strata 2. One stratum would contain all study subjects who are in a categorized age group and have the same race and gender (e.g., white females 30-40 years old). Q13.10 1. True 2. False – both the stratum-specific and the overall summary statistics will typically involve computing a point estimate, a confidence interval, and a test of hypothesis. 3. False – when carrying out stratum-specific analyses, the point estimate is a simple point estimate calculated for a specific stratum. 4. True 5. True 6. False – a Mantel-Haenszel test is not appropriate if there is significant and meaningful interaction over the strata.

CHAPTER

14

HOW TO DEAL WITH LOTS OF TABLES? STRATIFIED ANALYSIS
This is an analysis option for the control of extraneous variables that involves the following steps: 1. Categorize all variables. 2. Form combinations of categories (i.e., strata). 3. Perform stratum-specific analyses. 4. Perform overall E-D assessment if appropriate. Both stratum-specific analyses and overall assessment require a point estimate, an interval estimate, and a test of hypothesis. In this chapter we focus on overall assessment, which is the most conceptually and mathematically complicated of the four steps. For overall assessment, the point estimate is an adjusted estimate that is typically in the form of a weighted average of stratumspecific estimates. The confidence interval is typically a large-sample interval estimate around the adjusted (weighted) estimate. The test of hypothesis is a generalization of the Mantel-Haenszel chi square test.

An Example – 1 Control Variable
We illustrate the four steps of a stratified analysis with an example:

The tables below show data from a hypothetical retrospective cohort study to determine the effect of exposure to a suspected toxic chemical called TCX on the development of lung cancer. Suppose here that the only control variable of interest is smoking. First, we categorize this variable into two groups, smokers and non-smokers. Second, we form two-way tables for each stratum. Third, we perform stratum specific analyses as shown here. These data illustrate confounding. The crude data that ignores the control of smoking yields a moderately strong risk ratio estimate of 2.1. This is meaningfully different from the two estimates obtained when smoking is controlled, both of which indicate no association.

246 Chapter 14. How to Deal with Lots of Tables - Stratified Analysis

Step 3 also involves computing interval estimates and a P-value for each stratum and then interpreting the results separately for each stratum as well as for the crude data. Each stratum-specific analysis is essentially a simple analysis for a two-way table. Here are the computed results:

P = 0.25 Study Questions (Q14.1) 1. What is your interpretation of the stratum-specific results? 2. Does there appear to be interaction due to smoking? 3. Does there appear to be an overall effect of TCX exposure after controlling for smoking status? Step 4, the overall E D assessment, should only be performed when appropriate. When evidence of confounding is present, this assessment should be conducted. However, when there is sufficient evidence of interaction or effect modification, this step is considered inappropriate. In our example, the risk ratio estimates for both smoking groups are essentially the same, which indicates that it is reasonable to go ahead with a summary or overall assessment. To perform this step, we must do three things: compute an overall adjusted estimate of the exposure-disease effect over all the strata, carry out a test of hypothesis of whether or not there is an overall effect controlling for the stratification, and compute and interpret an interval estimate around the adjusted point estimate.

A Pocket Guide to Epidemiology 247

The adjusted estimate typically is some form of weighted average of stratumspecific estimates. The test procedure is the Mantel-Haenszel test for stratified analysis. The interval estimate is typically computed as a large sample confidence interval based on percentage points of the normal distribution. These three components of overall assessment will be described further in the activities to follow. Study Questions (Q14.1) continued A precision-based adjusted risk ratio estimate of the TCX to lung cancer relationship is computer to be 1.25. A 95% confidence interval around this estimate turns out to be (.78, 2.00). The MantelHaenszel test statistic has a P-value of .28. 4. What do you conclude from these results about the overall assessment of the ED relationship in this study? Summary The simplest form of stratification occurs when there is a single dichotomous variable to be controlled. In this case, only one variable is categorized (step 1) and two strata are obtained (step 2). Step 3 typically involves computing a point estimate, an interval estimate, and a P-value for each stratum. Overall assessment (step 4) may not be appropriate if there is interaction/effect modification. Step 4 involves computing an overall adjusted estimate of effect, a largesample confidence interval for the adjusted effect, and a test of significance (the Mantel-Haenszel test).

Overall Assessment?
Because the risk ratio estimates for both smoking groups are essentially the same, we have concluded that it is reasonable to go ahead with an overall assessment using an adjusted estimate, a confidence interval around this adjusted estimate, and a Mantel-Haenszel test for the stratified data. The results are presented below. They clearly indicate that there is no meaningful or significant effect of TCX on the development of lung cancer when controlling for smoking.

248 Chapter 14. How to Deal with Lots of Tables - Stratified Analysis

But, what if we obtained a different set of stratum specific estimates, for example, the results shown below (examples 2 and 3)? Would we still want to compute an adjusted estimate, obtain a confidence interval around it and compute a Mantel-Haenszel test?

Note: The rows of risk ratio results are, from top to bottom, examples 1, 2, and 3, respectively.

These two examples show a very strong interaction due to smoking. And, the type of interaction in example 2 is quite different from the interaction in example 3. The stratum-specific risk ratio estimates of 0.52 and 3.5 in example 2 are on the opposite side of the null value of 1. In contrast, the stratum-specific risk ratio estimates of 1.1 and 4.2 from example 3 are on the same side of the null value, although they are also quite different. When stratum specific effects are on opposite sides of 1, as in example 2, it is possible that they can cancel each other in the computing of an adjusted effect. Consequently, in this situation, the use of such an adjusted estimate, corresponding confidence interval, and Mantel-Haenszel test is not recommended. The important results in this case are given by the contrasting stratum-specific effects, and these are likely to be masked by carrying out overall assessment. When stratum specific effects are all in the same direction, as in example 3, a spurious appearance of no association cannot arise from cancellation of opposite effects. It may therefore be worthwhile, despite the interaction, to perform overall assessment, depending on the investigator 's judgment of how large the difference between stratum-specific effects is or how stable these estimates are. Summary Overall assessment (step 4) may not be appropriate if there is interaction/effect modification. The most compelling case for not carrying out an overall assessment is when significant stratum-specific effects are on opposite sides of the null value. When all stratum-specific effects are on the same side of the null value, overall assessment may be appropriate even if there is interaction. The most appropriate situation for performing overall assessment is when stratum-specific effects are all approximately equal, indicating no interaction over the strata.

A Pocket Guide to Epidemiology 249

An Example – Several Explanatory Variables
In recent years, antibiotic resistance has become a major problem in the treatment of bacterial infections. Many antibiotics that used to provide effective treatment against certain bacteria, particularly Staphylococcus aureus, or Staph, no longer work because newer strains of Staph aureus are resistant to antimicrobial drugs. When someone is diagnosed with infection due to Staph aureus, the first line of treatment typically involves methicillin-based antimicrobial drugs. However, strains of Staph aureus resistant to those drugs are now are considered a major problem for patients seen in emergency rooms. Resistant bacteria of this type are called methicillin-resistant Staph infections or MRSA. We may wonder what are the characteristics or risk factors associated with having an MRSA infection? To study this question, a cross-sectional study was carried out at Grady Hospital in Atlanta, Georgia involving 297 adult patients seen in an emergency department whose blood cultures taken within 24 hours of admission were found to have Staph aureus infection. Information was obtained on several variables, some of which were previously described risk factors for methicillin resistance: We use this information to illustrate a stratified analysis to assess whether previous hospitalization is associated with methicillin resistance, controlling for age, gender, and prior use of antimicrobial drugs. Age is continuous so we will categorize age into two groups (1=age greater than 55 years; 0=age less than or equal to 55 years). We first consider the crude data relating previous hospitalization to MRSA status:

Study Questions (Q14.2) 1. Looking at the crude data only, is previous hospitalization associated with methicillin resistance? 2. What reservations should you have about your answer to question 1? 3. Should you automatically stratify on age, sex, and prior antimicrobial use since they were measured in the study? 4. Since there were 297 persons in the study, why does the overall total equal 292? Now let 's see what happens when we stratify separately on age, sex, and prior antimicrobial drug use? Each stratified table is depicted separately in the

250 Chapter 14. How to Deal with Lots of Tables - Stratified Analysis following.
Relation between MRSA and Previous Hospitalization Stratified on Age

5. Focusing on stratifying age only, does there appear to be interaction/effect modification due to age based on the stratum-specific results? 6. The Breslow Day (BD) Test for Interaction provides a P-value for testing the null hypothesis that there is no interaction over the strata. Based on this test with stratifying on age, is there evidence of interaction? 7. Based on your answers to the above questions, is an overall assessment of the E-D relationship appropriate when stratifying on age?

ˆ 8. Is there confounding due to age? (Hint: cOR = 11.67.) 9. Does there appear to be a significant effect of previous hospitalization on MRSA when controlling for age? 10. What does the confidence interval for the adjusted estimate say about this estimate?
Relation between MRSA and Previous Hospitalization Stratified on Sex

11. Is an overall assessment of the E-D relationship appropriate when stratifying only on gender?

ˆ 12. Is there confounding due to gender? (Hint: cOR = 11.67.) 13. Does there appear to be a significant effect of previous hospitalization on MRSA status when controlling for gender?

A Pocket Guide to Epidemiology 251 Relation between MRSA and Previous Hospitalization Stratified on Prior Antimicrobial Drug use (“PAMDU”)

14. Is an overall assessment of the E-D relationship appropriate when stratifying only on PAMDU?

ˆ 15. Is there confounding due to PAMDU? (Hint: cOR = 11.67.) 16. Does there appear to be a significant effect of previous hospitalization on MRSA status when controlling for PAMDU?
Summary When several variables are being controlled using stratified analysis, the typical first step in the analysis is to analyze and interpret the crude data. The next step typically is to stratify separately on each control variable including carrying out an overall assessment of the E-D relationship, if appropriate. One approach to determine whether overall assessment is appropriate is to assess whether stratum-specific effects are more or less the same. Another approach is to carry out a Breslow Day test of the null hypothesis that there is no interaction/effect modification due to the variable(s) being stratified.

An Example – Several Explanatory Variables (Continued)
Here is a summary table that results from stratifying on each control variable separately.

252 Chapter 14. How to Deal with Lots of Tables - Stratified Analysis Study Questions (Q14.3) 1. Based on the information in the above table, which, if any, of the variables age, gender, and previous antimicrobial drug use needs to be controlled? 2. Is there a gain in precision from the control of any of the variables age, gender, and previous antimicrobial drug use? We now add to the summary table the results from controlling for two and three variables at a time.

3. Does controlling for age, gender, or both have an affect on the results after already controlling for previous antimicrobial drug use (PAMDU 4. Using the BD P-value, is there any evidence that there is interaction when stratifying on any or all of these three variables being controlled? 5. Why do you think it is necessary to use a correction factor such as .5 in strata that contain a zero frequency? 6. Based on all the information in the table, what is the most appropriate estimate of the odds ratio of interest? (You may choose two alternatives here.) 7. Is there evidence that previous hospitalization has an effect on whether or not a person is methicillin resistant to Staph aureus? The stratum-specific results when simultaneously controlling for age, gender, and previous antimicrobial drug use are shown in the box at the end of this chapter. There are 8 strata, because three variables are being controlled and each variable has two categories. Study Questions (Q14.3) continued (Note: there is no question 8) 9. What is the most obvious characteristic that describes the stratified results just shown? 10. What does your answer to the previous question indicate about stratumspecific analyses with these strata? 11. Based on comparing stratum-specific odds ratio estimates, does there appear to be interaction within the stratified data? 12. Give three reasons that justify doing an overall Mantel-Haenszel test using these data?

A Pocket Guide to Epidemiology 253 Summary When several variables are being controlled simultaneously using stratified analysis, not all of these variables may need to be controlled depending on whether a variable contributes to confounding or precision. The simultaneous control of several variables typically leads to strata with small numbers and often zero cell frequencies. When there are small numbers in some strata, stratum-specific conclusions may be unreliable. There are three things to consider when assessing interaction in stratified data: o Are stratum-specific estimates essentially the same? o Is the Breslow-Day test for interaction significant? o Are stratum-specific estimates unreliable because of small numbers?
Stratum Specific Results Here are the stratum specific results when simultaneously controlling for age, gender, and previous antimicrobial drug use. 1. Age < 55, Male, PAMDU=Yes Prev. Hosp. Yes No MRSA Yes 37 2 No 22 7 59 9 ˆ OR =5.89 3. Age < 55, Female, PAMDU=Yes Prev. Hosp. Yes No MRSA Yes 9 0 No 14 3 23 3 ˆ OR =4.59* 5. Age > 55, Male, PAMDU=Yes Prev. Hosp. Yes No MRSA Yes 24 1 No 2 2 26 3 ˆ R =24.00 O 7. Age > 55, Female, PAMDU=Yes Prev. Hosp. Yes No MRSA Yes 22 0 No 9 1 31 1 ˆ OR =7.11* *with .5 adjustment 2. Age < 55, Male, PAMDU = No Prev. Hosp. Yes No MRSA Yes 5 4 No 13 49 18 53 ˆ OR =4.71

39 29 68

9 62 71

9 17 26

4. Age < 55, Female, PAMDU = No Prev. Hosp. Yes No MRSA Yes 0 0 0 No 2 13 15 2 13 15 ˆ OR =5.4* 6. Age > 55, Male, PAMDU = No Prev. Hosp. Yes No MRSA Yes 2 2 No 7 12 9 14 ˆ OR =1.71

25 4 29

4 19 23

22 10 68

8. Age > 55, Female, PAMDU = No Prev. Hosp. Yes No MRSA Yes 3 3 6 No 5 15 20 8 18 26 ˆ OR =3.00

254 Chapter 14. How to Deal with Lots of Tables - Stratified Analysis Quiz (Q14.4) Label each of the following statements as True or False. 1. Stratification only involves categorizing variables into two groups and conducting separate analysis for each group. ??? 2. One of the four steps of a stratified analysis is to compute an overall summary E-D assessment when appropriate. ??? 3. The calculation of an overall summary estimate may be considered inappropriate if there is considerable evidence of statistical interaction. ??? 4. The calculation of an overall summary estimate may be considered inappropriate if there is considerable evidence of confounding. ??? 5. When considering the appropriateness of computing overall summary results, the investigator must exercise some judgment regarding the clinical importance of the observed differences among stratum-specific estimates as well as to the stability of these estimates. ??? 6. Compare the stratum specific RR estimates for each of the three situations below. Fill in the blank with yes, no or maybe regarding the appropriate use of a summary estimate. Situation: Opposite direction Same direction Uniform effect RR: Stratum 1 0.7 1.5 2.3 RR: Stratum 2 3.5 4.8 2.9 Overall Est. ??? ??? ???

See Lesson pages 14-2 through 14.6 in the ActivEpi CD ROM for a detailed discussion of the following topics: The Mantel-Haenszel (MH)Chi Square Test for Stratified Analysis When Not to Use the MH Test The MH Test for Person-time Cohort Studies Overall Assessment Using Adjusted Estimates Mantel-Haenszel Adjusted Estimates (e.g., mOR) Interval Estimation of Adjusted Estimates

References
Clayton DG. Some odds ratio statistics for the analysis of ordered categorical data. Biometrika 1974;61:525-31. Kleinbaum DG, Kupper LL, Morgenstern H. Epidemiologic Research: Principles and Quantitative Methods. John Wiley and Sons Publishers, New York, 1982. Mantel N. Chi-square tests with one degree of freedom: Extensions of the Mantel-Haenszel procedure. J Am Stat Assoc 1963;58:690-700. Mantel N, Haenszel W. Statistical aspects of the analysis of data from retrospective studies of disease. J Natl Cancer Inst 1959;22(4):719-48. Rezende NA, Blumberg HM, Metzger BS, Larsen NM, Ray SM, and McGowan JE, Jr. Risk factors for methicillin-resistance among patients with Staphylococcus aureus bacteremia at the time of hospital admission. Am J Med Sci 2002; 323(3):117-23.

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Answers Study Questions and Quizzes
Q14.1 1. For both smokers and non-smokers separately, there appears to be no association between exposure to TXC and the development of lung cancer. Never the less, it may be argued that the RR of 1.3 for smokers indicates a moderate association; however, this estimate is highly non-significant. 2. No, the two stratum-specific risk ratio estimates are essentially equal. Again, the RR of 1.3 for smokers indicates a small effect, but is highly nonsignificant. 3. No, even though the crude estimate of effects is 2.1, the correct analysis requires that smoking be controlled, from which the data show no effect of TCX exposure. An adjusted estimate over the two strata would provide an appropriate summary statistic that controls for smoking. 4. Since the adjusted point estimate is close to the null value of 1 and the Mantel-Haenszel test statistic is very non-significant, you should conclude that there is no evidence of and E-D relationship from these data Q14.2 1. Yes, the odds ratio of 11.67 is very high and the MH test is highly significant and, even though the confidence interval is wide, the interval does not include the null value. 2. The association may change when one or more variables are controlled. If this happens and the control variables are risk factors, then an adjusted estimate or estimates would be more appropriate. 3. Not necessarily. If one or more of these variables are not previously known risk factors for MRSA status, then such variables may not be controlled. 4. Some (n=5) study subjects had to having missing information on either MSRA status or on previous hospitalization information. In fact, it was on the latter variable that 5 observations were missing. 5. No, the stratum-specific odds ratios within different age groups are very close (around 11). 6. No, the P-value of .95 is very high, indicating no evidence of interaction due to age. 7. Yes, overall assessment is appropriate because there is no evidence of interaction due to age. 8. No, the crude and adjusted odds ratios are essentially equal. 9. Yes, the Mantel-Haenszel test for stratified data is highly significant (P