In an experiment I exposed each of 4 subject groups to a different condition to observe the impact on dependent variable. Assumptions for parametric tests were violated hence I decided to employ non-parametric Mann-Whitney for group comparisons. What I do not understand is why I have different scores of mean ranks in the output tables depending on comparison. For example, when I compare group 1 and 2, and then in a separate test group 1 and 3 the mean ranks for group 1 are different in output tables. When I run t-tests means remain the same in both tables. The conclusion I have reached is that it is due to the way Mann-Whitney is calculated but I also suspect I may have used the tests incorrectly. I would very much appreciate some help as I am unsure how to proceed. I have searched the forum for a similar query and could not find anything - apologies in advance if I omitted an answer to a similar question.