The Interpretation of Statistical Power after the Data have been Gathered

Post-hoc power estimates (power calculated for hypothesis tests after performing them) are sometimes requested by reviewers in an attempt to promote more rigorous designs. However, they should never be requested or reported because they have been shown to be logically invalid and practically misleading. We review the problems associated with post-hoc power, particularly the fact that the resulting calculated power is a monotone function of the p-value and therefore contains no additional helpful information. We then discuss some situations that seem at first to call for post-hoc power analysis, such as attempts to decide on the practical implications of a null finding, or attempts to determine whether the sample size of a secondary data analysis is adequate for a proposed analysis, and consider possible approaches to achieving these goals. We make recommendations for practice in situations in which clear recommendations can be made, and point out other situations where further methodological research and discussion are required.