CHOOSING A 2-SAMPLE LOCATION TEST

Type I error rate and power for the t test, Wilcoxon-Mann-Whitney (U) test, van der Waerden Normal Scores (NS) test, and Welch-Aspin-Sattert hwaite (W) test were compared for two independent random samples drawn from nonnormal distributions. Data with varying degrees of skewness ( S) and kurtosis (K) were generated using Fleishman's (1978) power func tion. Five sample size combinations were used with both equal and uneq ual variances. For nonnormal data with equal variances, the power of t he U test exceeded the power of the t test regardless of sample size. When the sample sizes were equal but the variances were unequal, the t test proved to be the most powerful test. When variances and sample s izes were unequal, the W test became the test of choice because it was the only test that maintained its nominal Type I error rate.