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Hi, I have a question regarding designing matrices in fsl. I am trying to design a matrix for un-paired two groups comparison (patients vs. controls) with one nuisance factor (grey matter volume). I understand that I need two separate columns for patients and controls to enter the GMV values for each group in a separate column. Now, to demean the grey matter volumes do I have to use the grand mean (mean of both groups put together) or group specific means (demeaning patients value by patients' mean and controls by controls' mean)? Best wishes Seyed Sajjadi