expect to encounter those distributions and what they look like. We also include a description of some

of their properties and how they’re related to other distributions. Some of them are accompanied by a

small table of critical values, corresponding to statistical significance at α = 0.05.

Figuring Out How Many Participants You Need

Of all the statistical challenges a researcher may encounter, none seems to instill as much apprehension

and insecurity as having to estimate the number of participants needed for a study. While smaller

sample sizes mean less data collection work, you want to make sure your target sample size is large

enough so that in the end, your study has sufficient power. You want to conduct a study with a high

probability of yielding a statistically significant result if the hypothesized effect is truly present in the

population.

Because sample-size estimation is such an important part of the design of any research project,

this book shows you how to make those estimates for the situations you’re likely to encounter

when doing biological research. As we describe each statistical test in Parts 4, 5, 6, and 7, we

explain how to estimate the number of participants needed to provide sufficient power for that

test. In addition, Chapter 25 describes ten simple rules for getting a “quick and dirty” estimate of

the required sample size.