Improving interpretability: gamma as an alternative to R-2 as a measure ofeffect size

A traditional measure of effect size associated with tests for difference b etween two groups is the variance explained by group membership (R-2). If e xposure to a disease causes a small but long term deficit in performance, h owever, R-2 does not capture that cumulating effect. We propose an alternat ive statistic, gamma, based on the probability of an unexposed person outpe rforming an exposed person. Although gamma is also a point estimate, it mor e easily conveys what the cumulating effect of a deficit would be. We discu ss some of the advantages of this measure. Copyright (C) 2000 John WiIey & Sons, Ltd.