The Mann–Whitney U Test is a popular test for comparing two independent samples. It is a nonparametric test, as the analysis is undertaken on the rank order of the scores and so does not require the assumptions of a parametric test. It was originally proposed by Frank Wilcoxon in 1945 for equal sample sizes, but in 1947 H. B. Mann and D. R. Whitney extended it to unequal sample sizes (and also provided probability values for the distribution of U, the test statistic).

When the null hypothesis is true, and the ranks of the two samples are drawn from the same population distribution, one would expect the mean rank for the scores in one sample to be the same as the mean rank for the scores ...