Statistical power calculations

¹ Department of Statistics and Actuarial Science, The University of Iowa, Iowa City, IA 52242

^* To whom correspondence should be addressed. E-mail: russell-lenth{at}uiowa.edu.

	Abstract

This article focuses on how to do meaningful power calculations and sample-size determination for common study designs. There are three important guiding principles: 1) certain types of retrospective power calculations should be avoided, as they add no new information to an analysis; 2) effect size should be specified on the actual scale of measurement, not on a standardized scale, and 3) rarely can a definitive study be done without first doing a pilot study. Some simple examples as well as a complex example are given. Power calculations are illustrated using Java applets developed by the author.

Key Words: Statistical power, Sample size