There is a need for effect sizes that are readily interpretable by a broad audience. One index that might fill this need is [pi], which represents the proportion of scores in one group that exceed the mean of another group. The robustness of estimates of [pi] to violations of normality had not been explored. Using simulated data, three estimates of [pi] ([pi-hat][subscript direct], r, and r[subscript robust]) were studied under varying conditions of sample size, distribution shape, and group mean difference. This study demonstrated that r and r[subscript robust] were biased estimates of [pi] when data were nonnormal. We recommend that neither be used in estimating [pi] unless data are normally distributed.