The Mann-Whitney test is a nonparametric test that allows two groups or conditions
or treatments to be compared without making the assumption that values are normally
distributed. So, for example, one might compare the speed at which two different
groups of people can run 100 metres, where one group has trained for six weeks and
the other has not.

Requirements

Two random, independent samples

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

Scale of measurement should be ordinal, interval or ratio

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.