The wilcoxon rank sum test?

a.compares paired random samples from one population

b.may be used only for one sided alternatives

c. requires two independent samples

d.makes the assumption that samples are selected from normally distributed populations

Option (c) i.e. requires two independent samples

When comparing two independent samples when the outcome is not normally distributed and the samples are small, a nonparametric test is appropriate.

A popular nonparametric test which is used to compare outcomes between two independent groups is the Wilcoxon Rank Sum Test . The Wilcoxon Rank Sum Test, also known as the Mann Whitney U test, is used to test whether two samples are likely to derive from the same population (i.e. that the two populations have the same shape).