This document provides information on calculating effect sizes when comparing two means. It defines effect size as the extent to which a phenomenon is present in a population or how false the null hypothesis is. It lists several common effect size measures for comparing two independent groups (Cohen's d, Hedges' g, Glass' Delta), correlations (r, χ2), and analysis of variance (eta squared, omega squared, interclass correlation). Formulas are provided for Cohen's d when using a t-test for independent means. An example is given of comparing study habits between public and private school students using a t-test and reporting statistical output.