12.6 Wilcoxon Rank Sum Test

The two-sample non-parametric Wilcoxon rank sum test (equivalent to the Mann-Whitney test) is performed on the two specified samples. The null hypothesis is that the distributions are the same (i.e., there is no shift in the location of the two distributions) with an alternative hypothesis that they differ on location (based on median).

This test does not assume that the two samples are normally distributed but does assume they have distributions of the same shape.

If the p-value is less than 0.05 then we reject the null hypothesis and accept the alternative hypothesis, that the two samples have different medians, at the 95% level of confidence.

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