Retrospective Statistical Power: Fallacies and Recommendations

The calculation of statistical power after a study has been concluded is a highly controversial practice in quantitative research. Retrospective power in association with statistical nonsignificance presents special challenge to applied researchers in interpreting their statistical outcomes. The purposes of the present study are to review the current debate on retrospective power analysis, to examine the evidential basis of some myths associated with it, and to recommend some practical guidelines for quantitative researchers. I first briefly explain the theoretical concepts of prospective and retrospective power. I then describe three fallacies in misusing and abusing retrospective power. I conclude by providing some recommendations for improving the statistical practice associated with statistically nonsignificant findings.

Keywords: Retrospective power, Post hoc power, Observed power, Statistical testing, Effect size, Confidence interval