Mann-Whitney U test

A statistical test for a variable measured on an ordinal scale comparing the difference in the location of two populations based on observations from two independent samples. In the Mann-Whitney U test, the two samples are combined and the cases are ranked in order of increasing size. The test statistic U, is computed as the number of times a score from sample 1 or group 1 precedes a score from sample or group 2. if the samples are from the same population, the distribution of scores from the two samples in the rank list should be random.