Monika Wahi

Cheat Sheet / Updated 07-27-2024

To estimate sample size in biostatistics, you must state the effect size of importance, or the effect size worth knowing about. If the true effect size is less than the “important” size, you don’t care if the test comes out nonsignificant. With a few shortcuts, you can pick an important effect size and find out how many participants you need, based on that effect size, for several common statistical tests. All the graphs, tables, and rules of thumb here are for 80 percent power and α = 0.05. In other words, the guidance applies to calculating sample size you need in order to have an 80 percent chance of getting a p value that’s less than or equal to 0.05. If you want sample sizes for other values of power and α, use these simple scale-up rules: For 90 percent power instead of 80 percent: Increase N by a third (multiply N by 1.33). For α = 0.01 instead of 0.05: Increase N by a half (multiply N by 1.5). For 90 percent power and α = 0.01: Double N (multiply N by 2).