Mann Whitney test is a distribution free test which is used as an alternative to the Student's t-test for assessing that whether the two populations have the same median. The test statistic U is calculated from comparing each pair of the values, that is one from each group, scoring these pairs 1 or 0 depending upon whether the first group observation is higher or lower than the from the second group and summing the resulting scores over all the pairs. In the case of no ties then the following g case could be seen
 

here n is the number of observations in the smaller group and W is the rank sum statistic associated with the Wilcoxon's rank sum test. The two tests are hence equivalent. Tables giving critical values of test statistic are available, and for the moderate and large sample sizes, a normal approximation can be taken in use.