I've used the Mann-Whitney U-test in order to check for possible differences between two relatively small groups (12 subject in each) on 15 different items. I didn't have any pre-defined assumption about which items would be different. However this simple Mann-Whitney U-test yielded a significant different (P<0.05) for ALL items tested.
My question: if/what corrections for multiple comparisons should I use here? Does the fact that I have found a significant different in all items make any difference?