1. The Wilcoxon Mann Whitney test tests the null hypothesis that the distributions are the same. The alternative hypothesis is that the distributions are not the same. You've already examined the distributions and they don't "look" the same so there's good indication that the null will be rejected. This doesn't make the test invalid. 4.7 Mann Whitney U-Test. Also known as the Wilcoxon two-sample test, Wilcoxon test, Wilcoxon Mann Whitney test, Wilcoxon rank-sum test. It helps to compare two independent groups irrespective of the sample size in each. Equivalent of the parametric unpaired t-test. More sensitive than the median test You can use Mann-Whitney on the original data (with or without the zeros, depending on what makes sense). Just understand that it isn't a test of medians. It is a test of stochastic equality. That is, Ho is that the probablity of an observation in one group being higher than an observation in another group is 0.50. $\begingroup$ You can convert that back to an actual rank sum by multiplying back by the standard deviation of the sum of the ranks and adding back the mean (under the null in each case - which mean an standard deviation you use depends on the form of the statistic you want); the relationship to the Mann-Whitney is straightforward. There are questions on site that discuss the different forms So the approach would be to compute the difference pre-test and post-test for every subject of the group A and the group B, and we call this variable PrePost. The you can use the Wilcoxon test to compare the two results you obtained, the PrePost from people in the group A against the PrePost from people of the group B. Thanks for the swift answer! Note that the t does not test for whether the distribution of the differences is symmetric about zero, so the Wilcoxon signed rank test is not truly a non-parametric counterpart of the paired t test. The Mann-Whitney test, on the other hand, assumes that all the observations are independent of each other (no basis for pairing here!). .

what is wilcoxon mann whitney test