the scientific method

LAB EXERCISE: Scientific Investigation

Laboratory Objectives
After completing this lab topic, you should be able to:
1. Identify and characterize questions that can be answered through scientific investigation.
2. Define hypothesis and explain what characterizes a good scientific hypothesis.
3. Identify and describe the components of a scientific experiment.
4. Summarize and present results in tables and graphs.
5. Discuss results and critique experiments.
6. Design a scientific experiment.
7. Interpret and communicate results.

Introduction
Biology is the study of the phenomena of life, and biologists observe living systems and organisms, ask questions, and propose explanations for those observations. Science assumes that biological systems are understandable and can be explained by fundamental rules or laws. Scientific investigations share some common elements and procedures, which are referred to as the scientific method. Not all scientists follow these procedures in a strict fashion, but each of the elements is usually present. Science is a creative human endeavor that involves asking questions, making observations, developing explanatory hypotheses, and testing those hypotheses. Scientists closely scrutinize investigations in their field, and each scientist must present his or her work at scientific meetings or in professional publications, providing evidence from observations and experiments that supports the scientist’s explanations of biological phenomena.

EXERCISE I: PRACTICING THE SCIENTIFIC METHOD

Read the following example and answer the questions that follow.

INVESTIGATION OF THE EFFECT OF SULPHUR DIOXIDE ON
SOYBEAN REPRODUCTION

Agricultural scientists were concerned about the effect of air pollution, sulfur dioxide in particular, on soybean production in fields adjacent to coal-powered power plants. Based on initial investigations, they proposed that sulfur dioxide in high concentrations would reduce reproduction in soybeans. They designed an experiment to test this hypothesis (Figure 1). In this experiment, 48 soybean plants, just beginning to produce flowers, were divided into two groups, treatment and no treatment. The 24 treated plants were divided into four groups of 6. One group of 6 treated plants was placed in a fumigation chamber and exposed to 0.6 ppm (parts per million) of sulfur dioxide for 4 hours to simulate sulfur dioxide emissions from a power plant. The experiment was repeated on the remaining three treated groups. The no-treatment plants were placed similarly in groups of 6 in a second fumigation chamber and simultaneously exposed to filtered air for 4 hours. Following the experiment, all plants were returned to the greenhouse. When the beans matured, the number of bean pods, the number of seeds per pod, and the weight of the pods were determined for each plant. Figure 1. Experimental Design for soybean experiment. The experiment was repeated four times. Soybeans were fumigated for 4 hours.

Determining the Variables
Read the description of each category of variable; then identify the variable described in the preceding investigation. The variables in an experiment must be clearly defined and measurable. The investigator will identify and define dependent, independent, and controlled variables for a particular experiment.

a) The Dependent Variable
Within the experiment, one variable will be measured or counted or observed in response to the experimental conditions. This variable is the dependent variable. For the soybeans, several dependent variables are measured, all of which provide information about reproduction.
What are the dependent variables?

b) The Independent Variable
The scientist will choose one variable, or experimental condition, to manipulate. This variable is considered the most important variable by which to test the investigator’s hypothesis and is called the independent variable.
What was the independent variable?

Can you suggest other variables that the investigator might have changed that would have had an effect on the dependent variables?

Although other factors, such as light, temperature, time, and fertilizer, might affect the dependent variables, only one independent variable is usually chosen.
Why is it important to have only one independent variable?

Why is it acceptable to have more than one dependent variable?

c) Controlled Variables
Consider the variables that you identified as alternative independent variables. Although they are not part of the hypothesis being tested in this investigation, they would have significant effects on the outcome of this experiment. These variables must, therefore, be kept constant during the course of the experiment. They are known as the controlled variables. The underlying assumption in experimental design is that the selected independent variable is the one affecting the dependent variable. This is only true if all other variables are controlled.
What are the controlled variables in this experiment?
What variables other than those you may have already listed can you now suggest?

Choosing or Designing the Procedure
The procedure is the stepwise method, or sequence of steps, to be performed for the experiment. It should be recorded in a laboratory notebook before initiating the experiment, and any exceptions or modifications should be noted during the experiment. The procedures may be designed from research published in scientific journals, through collaboration with colleagues in the lab or other institutions, or by means of one’s own novel and creative ideas. The process of outlining the procedure includes determining control treatment(s), levels of treatments, and numbers of replications.

a) Level of Treatment
The value set for the independent variable is called the level of treatment. For this experiment, the value was determined based on previous research and preliminary measurements of sulfur dioxide emissions. The scientists may select a range of concentrations from no sulfur dioxide to an extremely high concentration. The levels should be based on knowledge of the system and the biological significance of the treatment level.
What was the level of treatment in the soybean experiment?

b) Replication
Scientific investigations are not valid if the conclusions drawn from them are based on one experiment with one or two individuals. Generally, the same procedure will be repeated several times (replication), providing consistent results. Notice that scientists do not expect exactly the same results inasmuch as individuals and their responses will vary. Results from replicated experiments are usually averaged and may be further analyzed using statistical tests.
Describe replication in the soybean experiment.

c) Control
The experiment design includes a control in which the independent variable is held at an established level or is omitted. The control or control treatment serves as a benchmark that allows the scientist to decide whether the predicted effect is really due to the independent variable.
What was the control treatment in this experiment?

What is the difference between the control and the controlled variables discussed previously?

EXERCISE II: DESIGNING AN EXPERIMENT

Introduction
In this exercise, the entire class, working together, will practice investigating a question using what you have learned so far about the scientific process.

a) Question
Cardiovascular fitness can be determined by measuring a person’s pulse rate and respiration rate before and after a given time of aerobic exercise. A person who is more fit may have a relatively slower pulse rate and a lower respiratory rate after exercise, and his or her pulse rate should return to normal more quickly than that of a person who is less fit. Your assignment is to investigate the effect of a well-defined, measurable, controllable independent variable on cardiovascular fitness.

Specific questions can be asked about an independent variable related to the broad topic of cardiovascular fitness. For example, your question might be “Does cigarette smoking have an effect on cardiovascular fitness?” List the questions in the space provided. Choose the best question and propose a testable hypothesis.

b) Hypothesis
Record the hypothesis chosen by the class.

The Experiment
A test, called the step test, that is often used for assessing cardiovascular fitness (Kusinitz and Fine, 1987). Here are the basic elements of this test:
1. The subject steps up and down on a low platform, approximately 8 in. from the ground, for 3 minutes at a rate of 30 steps per minute.
2. The subject’s pulse rate is measured before the test and immediately after the test. The subject should be sitting quietly when the pulse is counted. Use three fingers to find the pulse in the radial artery (the artery in the wrist, above the thumb). Count the number of beats per minute. (Count the beats for 30 seconds and multiply by 2.)
3. Additionally, the pulse rate is measured at 1-minute intervals after the test until the pulse rate returns to normal (recovery time). Count the pulse for 30 seconds, rest 30 seconds, count 30 seconds, and rest 30 seconds. Repeat this procedure until the pulse returns to normal. Record the number of minutes to return to the normal pulse rate. (Do not record the pulse rate.)

*As a group, design an experiment and record the components below:

Dependent variable(s):

Independent variable:

Controlled variables:

Control:

Level of treatment:

Replication:

Summarize the experimental designed by your class:

Prediction
Predict the results of the experiment based on your hypothesis (if/then).

Performing the Experiment
Following the procedures established by your investigative team, perform the experiment and record your results.

Results
Record total class results in a Table. Identify the treatment conditions at the top of the table.

Presenting and Analyzing Results

Once the data are collected, they must be organized and summarized so that the scientists can determine if the hypothesis has been supported or falsified. In this exercise, you will design tables and graphs; the latter are also called figures. Tables and figures have two primary functions. They are used (1) to help you analyze and interpret your results and (2) to enhance the clarity with which you present the work to a reader or viewer.

Tables
You have collected data from your experiment in the form of a list of numbers that may appear at first glance to have little meaning. Look at your data. How could you organize the data set to make it easier to interpret? You could average the data set for each treatment, but even averages can be rather uninformative. Could you use a summary table to convey the data (in this case, averages)?
Table 2 is an example of a table using data averages of the number of seeds per pod and number of pods per plant as the dependent variables and exposure to sulfur dioxide as the independent variable. Note that the number of replicates and the units of measurement are provided in the table and table legend.

Table 2. Effects of 4-Hour Exposure to 0.6 ppm Sulfur Dioxide on Average Seed and Pod Production in Soybeans.

Treatment Number Seeds per Pod Pods per Plant
Control 24 3.26 16
S02 24 1.96 13

Tables are used to present results that have a few too many data points. They are also useful for displaying several dependent variables. For example, average number of bean pods, average number of seeds per pod, and average weight of pods per plant for treated and untreated plants could all be presented in one table.

The following guidelines will help you construct a table: • All values of the same kind should read down the column, not across a row. Include only data that are important in presenting the results and for further discussion. • Information and results that are not essential (for example: test-tube number, simple calculations, or data with no differences) should be omitted. • The headings of each column should include units of measurement, if appropriate. • Tables are numbered consecutively throughout a lab report or scientific paper. For example” Table 4 would be the fourth table in your report. • The title, which is located at the top of the table, should be clear and concise, with enough information to allow the table to be understandable apart from the text. Capitalize the first and important words in the title. Do not capitalize articles (a, an, the), short prepositions, and conjunctions. • Refer to each table in the written text. Summarize the data and refer to the table; for example, “The plants treated with sulfur dioxide produced an average of 1.96 seeds per pod (Table 2).” Do not write, “See the results in Table 2.” • If you are using a database program, such as Excel, you should still sketch your table on paper before constructing it on the computer.

Application
1. Using the data from your experiment, design a summary table to present the results for one of your dependent variables, pulse rate. Your table need not be the same size or design as the sample. In your table, provide units of the dependent variable (pulse rate). Tell the reader how many replications (if any) were used to calculate the averages.
2. Compose a title for your table. Refer to the guidelines in the previous section.

Interpreting and Communicating Results

The last component of a scientific investigation is to interpret the results and discuss their implications in light of the hypothesis and its supporting literature. The investigator studies the tables and graphs and determines if their hypothesis has been supported or falsified. If the hypothesis has been falsified, the investigator must suggest alternate hypotheses for testing. If the hypothesis has been supported, the investigator suggests additional experiments to strengthen the hypothesis, using the same or alternate methods.
Scientists will thoroughly investigate a scientific question, testing hypotheses, collecting data, and analyzing results, until they are satisfied that they can explain the phenomenon of interest. The final phase of a scientific investigation is the communication of the results to other scientists. Preliminary results may be presented within a laboratory research group and at scientific meetings where the findings can be discussed. Ultimately, the completed project is presented in the form of a scientific paper that is reviewed by scientists within the field and published in a scientific journal. The ideas, procedures, results, analyses, and conclusions of all scientific investigations are critically scrutinized by other scientists. Because of this, science is sometimes described as self-correcting, meaning that errors that may occur are usually discovered within the scientific community.
Scientific communication, whether spoken or written, is essential to science. During this laboratory course, you often will be asked to present and interpret your results at the end of the laboratory period. Additionally, you will write components of a scientific paper for many lab topics.