Calculating Sample Size
Calculating Sample Size
Types of Samples
Subjective or Convenience Sample
- Has some possibility of bias
- Cannot usually say it is representative
- Selection made by ease of collection
Simple Random Sample
- No subjective bias
- Equal chance of selection; e.g., select the fifth chart seen on every third day
- Can usually be backed to say it is representative
Systematic Sample
- Is a random sample
- Equal chance of selection due to methodology; e.g., computer-generated list of random numbers, or every fifth name on a generated list
- Can usually be backed to say it is representative
Stratified Sample
- Breakdown the population into subgroups, then take a random sample from each subset
- Can usually be backed to say it is representative Sample Size Calculation
The Automated Method
If you know your population size and desired confidence level you may use this Web-based calculator to automatically calculate sample size.
The Manual Calculation Method
To perform sample size calculation manually, you need the following values:
Population Value: Size of the population from which the sample will be selected. (Number of users or number of encounters)
Expected Frequency of the Factor under Study always err toward 50%
Worst Acceptable Frequency
If 50% is the true rate in the population, what is the result farthest from the rate that you would accept in your sample? If your confidence interval were 4%, then your worst acceptable frequency would be 54% or 46%.
2. Formula: Sample Size = n / [1 + (n/population)]
In which n = Z * Z [P (1-P)/(D*D)]
P = True proportion of factor in the population, or the expected frequency value
D = Maximum difference between the sample mean and the population mean,
Or Expected Frequency Value minus (-) Worst Acceptable Value
Z = Area under normal curve corresponding to the desired confidence level
Confidence Level/ Value for Z
90% / 1.645
95% / 1.960
99% / 2.575
99.9% / 3.29
B. Population
Types of Samples
Subjective or Convenience Sample
- Has some possibility of bias
- Cannot usually say it is representative
- Selection made by ease of collection
Simple Random Sample
- No subjective bias
- Equal chance of selection; e.g., select the fifth chart seen on every third day
- Can usually be backed to say it is representative
Systematic Sample
- Is a random sample
- Equal chance of selection due to methodology; e.g., computer-generated list of random numbers, or every fifth name on a generated list
- Can usually be backed to say it is representative
Stratified Sample
- Breakdown the population into subgroups, then take a random sample from each subset
- Can usually be backed to say it is representative Sample Size Calculation
The Automated Method
If you know your population size and desired confidence level you may use this Web-based calculator to automatically calculate sample size.
The Manual Calculation Method
To perform sample size calculation manually, you need the following values:
Population Value: Size of the population from which the sample will be selected. (Number of users or number of encounters)
Expected Frequency of the Factor under Study always err toward 50%
Worst Acceptable Frequency
If 50% is the true rate in the population, what is the result farthest from the rate that you would accept in your sample? If your confidence interval were 4%, then your worst acceptable frequency would be 54% or 46%.
2. Formula: Sample Size = n / [1 + (n/population)]
In which n = Z * Z [P (1-P)/(D*D)]
P = True proportion of factor in the population, or the expected frequency value
D = Maximum difference between the sample mean and the population mean,
Or Expected Frequency Value minus (-) Worst Acceptable Value
Z = Area under normal curve corresponding to the desired confidence level
Confidence Level/ Value for Z
90% / 1.645
95% / 1.960
99% / 2.575
99.9% / 3.29
B. Population
References: AO. 1990. Conducting small-scale nutrition surveys: A field manual, Rome.Magnani, Robert. 1997. Sampling guide. IMPACT Food Security and Nutrition Monitoring Project, Arlington, Va.UNICEF. 1995. Monitoring progress toward the goals of the World Food Summit for Children: A practical handbook for multiple indicator surveys. New York. | |