Type I Error Probabilities of the Wilcoxon-Mann-Whitney Test and Student T Test Altered by Heterogeneous Variances and Equal Sample Sizes

The Student t test maintains its significance level more consistently than the Wilcoxon-Mann-Whitney test when variances of treatment groups are unequal and sample sizes are equal. The probability of a Type I error of the nonparametric test systematically increases above the nominal significance level as the ratio of population standard deviations increases, while that of the t test remains fairly stable.