rank sum test, a nonparametric statistical test for ordinal data, testing the null hypothesis that two samples are drawn from the same population against the alternative hypothesis that the two samples are drawn from two populations having probability distributions of the same shape but different locations. It is based on the value of the rank sum statistic, which is calculated as the sum of the ranks of each sample after the observations in both samples are jointly ranked in ascending order; if and only if the null hypothesis is true, the average ranks of the two samples will be similar. Also called Mann-Whitney test, Mann-Whitney U test.
