The Wilcoxon test is a nonparametric test designed to evaluate the difference
between two treatments or conditions where the samples are correlated. In particular,
it is suitable for evaluating the data from a repeated-measures design in a situation
where the prerequisites for a dependent samples t-test are not met. So, for example,
it might be used to evaluate the data from an experiment that looks at the reading
ability of children before and after they undergo a period of intensive training.

Requirements

Matched data

The dependent variable is continuous - in other words, it must, in principle, be
possible to distinguish between values at the nth decimal place

For maximum accuracy, there should be no ties, though this test - like others -
has a way to handle ties

Null Hypothesis

The null hypothesis asserts that the medians of the two samples are identical.