Chapter 26

Ten Easy Ways to Estimate How Many Subjects You Need

In This Chapter

arrow Quickly estimating sample size for several basic kinds of tests

arrow Adjusting for different levels of power and alpha

arrow Adjusting for unequal group sizes and for attrition during the study

Sample-size calculations tend to frighten researchers and send them running to the nearest statistician. But if you’re brainstorming a possible research project and you need a ballpark idea of how many subjects to enroll, you can use the ten quick and (fairly) easy rules of thumb in this chapter.

remember.eps Before you begin, look at Chapter 3, especially the sections on hypothesis testing and the power of a test, so that you have the basic idea of what power and sample-size calculations are all about. Think about the effect size of importance (such as the difference in some variable between two groups, or the degree of correlation between two variables) that you want to be able to detect. Then find the rule for the statistical test that’s appropriate for the primary objective of your study.

The first six sections tell you how many ...

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