Understanding statistical power

This article provides an introduction to power analysis so that readers hav e a basis for understanding the importance of statistical power when planni ng research and interpreting the results. A simple hypothetical study is us ed as the context for discussion. The concepts of false findings and missed findings are introduced as a way of thinking about type I and type II erro rs. The primary factors that affect power are described and examples are pr ovided. Finally, examples are presented to demonstrate 2 uses of power anal ysis, 1 for prospectively estimating the sample size needed to insure findi ng effects of a known magnitude in a study and 1 for retrospectively estima ting power to gauge the likelihood that an effect was missed.