Hi,
similar questions have been asked before, I know, but like many others I got confused studying the answers, so I have to ask again.
I have created skeletonized FA maps of 40 participants using TBSS and want to find out if 1 behavioral measure (range 19-71, mean 47.7674) is associated with these FA maps. In the FEAT user guide you recommend to enter the behavioral measure as an extra EV, which should be orthogonal wrt the group mean EV, i.e. demeaned.
I have run the analysis with both raw and demeaned behavioral scores and the results are very different.
Now my questions are:
1. Why are the results different given that the differences between the individual behavioral scores are the same?
2. Is the analysis really more valid when I use demeaned behavioral covariates? If yes, why? If not, why?
3. If I demean the behavioral covariate, don't I have to demean the FA maps as well?
Thanks for your patient answers,
Mike
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