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Dear Michael, You are right. The F-contrast testing for the interaction in 2x3 factorial design is: [1 -1 0 -1 1 0 0 1 -1 0 -1 1] All the contrasts can be given by calling: c = spm_make_contrasts([2 3]); Best regards, Guillaume. On 14/09/17 01:38, Michael L. wrote: > Hi, > > I have a question about the contrast weights in flexible factorial. In Glascher and Gitelman's paper they use the example of a 2x3 within-subjects design with columns A1B1 A1B2 A1B3 A2B1 A2B2 A2B3. The interaction A x B is defined as -1 0 1 1 0 -1, but isn't this the interaction between the two levels of factor A (A1 and A2) and two levels of factor B (B1 and B3)? > Thank you. > > Best regards, > Michael > -- Guillaume Flandin, PhD Wellcome Trust Centre for Neuroimaging University College London 12 Queen Square London WC1N 3BG