Effect Size FAQs

By some counts there are more than 70 effect size indexes. Some of them you will be familiar with (e.g., odds ratio, relative risk). Some double-up as test statistics (e.g., r, R²). And others sound like planets from Star Trek (e.g., the Pillai-Bartlett V).

Most effect size indexes can be grouped into one of two families:

1. differences between groups, a.k.a the d family (e.g., risk difference, risk ratio, odds ratio, Cohen’s d, Glass’s delta, Hedges’ g, the probability of superiority)
2. measures of association, a.k.a. the r family (e.g., the correlation coefficient r, R², Spearman’s rho, Kendall’s tau, phi coefficient, Cramer’s V, Cohen’s f, η²)

For more, see my book Effect Size Matters: