What is it?The effect size is a quantitative measure of the magnitude of effect of a factor on an outcome.
What does it do?The p-value on its own just tells you whether the difference or assiocation between two or more variables is statistically significant or not. It does not tell you how large or how strong the difference or association is. Effect size is able to provide that information, as it measures either the size of a difference between groups or the strength of associations between groups. The larger the effect size, the stronger the relationship or difference between two variables.
Useful to knowIt is often thought that a lower p-value means that the difference or association is larger or stronger. This is not true, as this can only be expressed in the effect size.
April 27, 2021