Variables often show evidence of clustering at extreme values and of graininess, that is, of a limited number of distinct values. Scores on two subscales of a quality-of-life measure, traditionally analyzed with OLS regression or ANOVA models, provide examples. Ignoring or failing to detect such features of the data will result in poor estimates of effect size.
ConroyR. M.2002. Choosing an appropriate real-life measure of effect size: the case of a continuous predictor and a binary outcome. Stata Journal2(3): 290–295.