I am trying to work out is there is any association between occurrence or not of an event in around 500 individuals and their randomised binary grouping (intervention vs placebo). However I also want to see if this holds true when broken down by one of 5 locations in which the individuals were treated.
A chi square test on the entire group shows a significant result. However if I repeat the test on each of the 5 subgroups only one subgroup is significant (<0.05) and all of the others are non-signficant.
Further still I have then corrected for multiple testing by multiplying the p-values by 6 (5 tests for each centre and then 1 test for entire group). This does not affect the subgroups but does affect the overall group which is no longer significant after multiple testing correction. I know this issue is controversial. Had I just stuck with the original hypothesis and performed one chi square test I would be saying it is significant but once I do subgroup testing I am then having to say this is not the case. Does anyone have any suggestions on how to approach this problem?