14 PART 1 Getting Started with Biostatistics

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 partici-

pants 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.