Power of the independent samples t test under a prevalent psychometric measure distribution.

Studies in psychology often have low power because of inadequate sample size. Thus, recent articles in this journal have suggested making sample size determinations through readily available tables that are based on population normality. Questions have been raised on the use of these power tables because prevalent psychometric distributions, such as the discrete mass at zero with gap that occurs with first use or onset variables, are radically nonnormal. In addition to demonstrating the robustness of the independent samples t test with respect to type I error, the major finding of this study shows that researchers may use these power tables without modification for this radically nonnormal distribution.