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Dear SPMers, I would like to compare two groups of subjects ( 31 females and 50 males) using a flexible factorial design, with a 2 x 2 factorial design (two groups and two conditions), and a covariate (total GM volume). I have set up the design following the tutorial by Glascher and Gitelman (two groups of subjects, design 1) and have run the following contrasts: main effect of condition: [zeros( 1, 81 ) 1 -1 1 -1] [zeros( 1, 81 ) -1 1 -1 1] With this contrast the results make sense. Then I have run the following: interaction group x condition: [zeros( 1, 81 ) 1 -1 -1 1] [zeros( 1, 81 ) -1 1 1 -1] Are these contrasts correct if I want to test group x condition interaction? Finally I have tried to assess the main effect of group using the following contrasts: [ones( 1, 31 )/31 -ones( 1, 50 )/50 0.5 0.5 -0.5 -0.5] [-ones( 1, 31 )/31 -ones( 1, 50 )/50 -0.5 -0.5 0.5 0.5] These contrasts only work when I completely remove the covariate (GM volume) from the design. I cannot figure out how to assess for differences between group while including the covariate in the model. Thanks a lot for your time, Vittoria