Interpreting Average Effect Sizes: Never a Center without a Spread

School leaders today are making important decisions regarding education innovations based on published average effect sizes, even though few understand exactly how effect sizes are calculated or what they mean. This article explains how average effect sizes are determined in meta-analyses and the importance of including measures of variability with any average effect size. By considering the variation in effect sizes among studies of the same innovation, education leaders can make better decisions about innovations and greatly increase the likelihood of achieving optimal results from implementation.