To do a sample size calculation, you can use the online sample size calculator available at: http://www.math.uiowa.edu/~rlenth/Power/
Worker example
Scenario 1
'S-Syndrome (SS)' is characterized by profound irritability, disorientation and fatigue for those infected individuals. The efficacy of a vaccine (called 'BG vaccine') in preventing adulthood SS remains uncertain, and a study is designed to compare the vaccination coverage rates in a group of MPH students infected with SS and a group of controls with equal sample size. Available information indicates that approximately 30% of the controls are vaccinated. The primary investigator plans to have an 80% chance of detecting an odds ratio significantly different from 1 at the 5% level of significance. If an odds ratio of 2 would be considered an important difference between the two groups, what should the sample size be included in each study group?
Assumptions
Level of significance: 0.05
Statistical power required: 0.8
This can be rearranged as
?Sample size calculations
Enter p1=0.462, p2=0.3, alpha=0.05.
Adjust sample size until reaching desired power.
Sample size in each group: 152
Total sample size: 304
Scenario 2
If number of cases is limited to 100, untick 'Equal ns', set n1=100, and increase n2 until the power reaches 80%.
The required sample sizes are 100 cases and 293 controls to reach 80% power for OR of 2.
If effect sizes smaller than OR = 2 are of interest, the sample size would be larger. Use the formula shown previously to calculate p_{1}, based on particular values of p_{2} and OR.
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