What are the two families of effect size?

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: