that is used to compare two sample means that come from the same population, and used to test whether two sample means are equal or not. Mann-Whitney U test is equivalent to Wilcoxon rank-sum test. The computations for the Mann–Whitney U test that are now described are based on the formula for the case where both nis are less than or equal to 20. The total number of subjects across groups is N = n1 + n2 and U = n 1 ⋅ n 2 + n 1 ( n 1 + 1 ) 2 − R 1 , where R1 is the sum of the ranks for group 1. Disadvantages. This test is mainly designed for comparing the distributions of two independent samples or groups. It cannot directly compare more than two groups. Extremely imbalanced sample sizes can negatively impact the test's power as the test's ability to detect significant differences can get reduced.