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Dear all, when correcting for total intracranial volume (TIV), should I treat TIV as a Covariate or as Global Value for the Global Calculation (as suggested in the VBM Tutorial). I did both analyses (TIV as Covariate or as Global Value) and the results are different. TIV as Covariate seems to increase the statistical power to detect group differences compared to TIV as Global Value. I would be very grateful for your suggestions on this subject. Silke Dr. Dipl.-Psych. Silke Matura Laboratory of Neuroscience Department of Psychiatry, Psychosomatic Medicine and Psychotherapy University of Frankfurt Heinrich-Hoffmann Str. 10 60528 Frankfurt am Main Tel. 069-6301- 83839 [log in to unmask]<mailto:[log in to unmask]>