This is used to test the hypothesis that two independent samples have come from the same population. Because it is non-parametric, it makes only limited assumptions about the distribution of the data. The method employed is a sum-of-ranks comparison. It is an alternative to the two sample t-test, and is based only on the order in which the observations from the two samples fall. It is of evident use when data are ordinal in nature and a comparison of means is meaningless. See Frank Wilcoxon (1945), 'Individual comparisons by ranking methods', Biometrics, 1, 80-83.
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