Improving interpretability: Gamma as an alternative to R-2 as a measure of effect size

A traditional measure of effect size associated with tests for difference between two groups is the variance explained by group membership (R-2). If exposure to a disease causes a small but long term deficit in performance, however, R-2 does not capture that cumulating effect. We propose an alternative statistic, gamma, based on the probability of an unexposed person outperforming an exposed person. Although gamma is also a point estimate, it more easily conveys what the cumulating effect of a deficit would be. We discuss some of the advantages of this measure. Copyright (C) 2000 John WiIey & Sons, Ltd.