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Hi all, Sorry for asking this question again, but I haven’t found any answers even in the maths books… I am doing a VBM analysis comparing 2 groups. Unfortunately, group 1 is significantly older than group 2. When NOT using age as covariate there are significantly gray differences between these two groups (which also make sense). But the factor group also correlates significantly with age (as expected) Using age as a single covariate the significant differences in the gray matter disappear. When using the covariate option interaction and centering around factor1 , the significant differences are back again. Is it allowed to use this option even if I am only looking at the group differences (there are now 2 covariate regressors, one for each group)? Many thanks boris Priv.-Doz. Dr. Boris Suchan Institute of Cognitive Neuroscience Ruhr University Bochum Universitätsstr. 150 44780 Bochum GAFO 05/613 Tel.: + 49 234 3227575 Fax: + 49 234 3214622 mailto: <mailto:[log in to unmask]> [log in to unmask] <http://www.ruhr-uni-bochum.de/neuropsy/mitarbeiter/boris_suchan.html> http://www.ruhr-uni-bochum.de/neuropsy/mitarbeiter/boris_suchan.html