mann-whitney U test? plz help math n00b? I am trying to test the difference in motor performance (measured by the amount of time each rat is able to keep running on a moving beam) between two groups of rats (those with early exposure to a common antidepressant and those with early exposure to Saline as control) in SPSS. There are three rats in each group and over a hundred entries for each of the six rats. I feel that the Mann-Whitney U test is most appropriate to determine the significance of my data. The problem is that I do not think that this takes into account the measures for each particular rat. One of these rats is an ENORMOUS outlier, big enough that it may make the entire group look significantly different if the program does not differentiate between rats within a group and only looks at all the times for each group as a whole. I understand that non-parametric tests are suited to deal with huge variation within a group but my concern is whether or not the mann-whitney U test is suited to deal with consistently large variation coming from one or two samples with hundreds of entries.