I am using randomise to analyse whole brain correlations between DTI measures (e.g. FA) and other measures (e.g. cognitive scores). Since data acquisition was performed on two scanners I want to include scanner site as a dummy variable.
Regressors of interest should be demeaned in this type of analyses, should a regressor of no interest (scanner site dummy variable)also be demeaned? When comparing two analyses, one with the demeaned dummy variable and one with the 'normal' dummy variable, I found some differences.
I hope that someone here can tell me which method to use (and why).