The Wilcoxon rank sum test is a nonparametric test that may be used to assess whether the distributions of observations obtained between two separate groups on a dependent variable are systematically different from one another. Developed by Frank Wilcoxon in 1945, the test replaces the obtained values of a dependent variable with a rank score, and inferentially tests whether the sum of the ranks within each group is likely to be obtained by chance from the same population. As the test does not use group means and standard deviations as estimations of population parameters, it is not beholden to the assumption of normality. It is therefore considered an excellent nonparametric alternative to the independent t test in cases where populations might not be normally distributed ...
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