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*, *R*^{2}). 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:

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

For more, see my book* Effect Size Matters:*