Wilcoxon's ranksum test is the distribution free method or technique used as an alternative to the Student's t-test for assessing whether two populations have the same location. Given the sample of observations from each population, all observations are ranked as if they were from the single sample and the test statistic Wis the sum of ranks in the smaller group. Equivalent to Mann-Whitney test. The tables giving critical values of test statistic are available, and for the moderate and large sample sizes, a normal estimate can be used.