This document discusses effect size, which is a measure used to quantify the size of the difference between two groups. It is calculated by taking the difference between the means of two groups and dividing by the standard deviation. Effect sizes of 0.2, 0.5, and 0.8 are considered small, medium, and large, respectively. Meta-analyses often use effect sizes to combine results from different studies by standardizing outcomes. Visible Learning, a meta-analysis of 800 other meta-analyses, found that most educational interventions have a small positive effect size of around 0.4, which corresponds to a two year increase in achievement. Effect sizes are useful for comparing impacts across different studies and determining what types of teaching practices have