How to Estimate and Interpret Various Effect Sizes
The present article presents a tutorial on how to estimate and interpret various effect sizes. The 5th edition of the Publication Manual of the American Psychological Association (2001) described the failure to report effect sizes as a “defect” (p. 5), and 23 journals have published author guidelines requiring effect size reporting. Although dozens of effect size statistics have been available for some time, many researchers were trained at a time when effect sizes were not emphasized, or perhaps even taught. Consequently, some readers may appreciate a review of how to estimate and interpret various effect sizes. In addition to the tutorial, the authors recommend effect size interpretations that emphasize direct and explicit comparisons of effects in a new study with those reported in the prior related literature, with a focus on evaluating result replicability. For decades, statistical significance has been the norm for evaluating results. In fact, little change has occurred since Carver (1993) noted: “A quick perusal of research journals, educational and psychological statistic textbooks, and doctoral dissertations will confirm that tests of statistical significance continue to dominate the interpretation of quantitative data in social science research” (p. 294). Although statistical significance “evaluates the probability or likelihood of the sample results, given the sample size, and assuming that the sample came from a population in which the null hypothesis is exactly true” (Thompson, 2003, p. 7), statistical