What is effect size?
Effect size is a measure of how meaningful the relationship between variables or the difference between groups is. Effect size is unrelated to significance, meaning that we cannot determine if a relationship or difference is significant by looking at an effect size. In fact, we don't even consider effect size unless the statistical test has significant results.
There are many different measures of effect size, depending on what statistical test you are conducting. All parametric and non-parametric tests have at least one appropriate measure of effect size, but many have more than one that could be used.
The purpose of this guide is to help you get familiar with common measures of effect size. Use the tabs above to learn more about each measure.