Mann-Whitney U Test
A non-parametric test comparing two independent groups by ranking all observations, used as an alternative to the independent-samples t-test.
The Mann-Whitney U test compares two independent groups without assuming normally distributed data. All observations are pooled and ranked, and the test asks whether values in one group tend to be higher than in the other. It is often described as a test of medians, though strictly it assesses whether one group tends to outrank the other. It is the usual alternative to the independent-samples t-test when data are ordinal, skewed, or the sample is small.
The test still assumes independent observations, and interpreting it as a comparison of medians is cleanest when both groups have similarly shaped distributions.
In a thesis, reach for this test when normality checks fail on a two-group comparison. Report the U statistic, the p-value, the group medians, and an effect size, and explain in the methodology chapter why the parametric option was not appropriate.
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