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Hi BaSti, great to hear from you and many thanks for your very helpful suggestions. After reading your posting I immediately realized that I over specified my model which makes it senseless to test the contrast I defined. However, a small problem I still have is that I cannot test for group main effects in the model you proposed. My regressors are the following (G=Group, C=Condition): G1C1 G1C2 G2C1 G2C2. I can test for condition main effects (1 -1 1 -1 and -1 1 -1 1) as well as for interactions (1 -1 -1 1 and -1 1 1 -1) but the group main effects (1 1 -1 -1 or -1 -1 1 1) do not work. I am sure it is again a very simple mistake but right now, I don't know which. Cheers, Peter -----Ursprüngliche Nachricht----- Von: [log in to unmask] [mailto:[log in to unmask]] Gesendet: Donnerstag, 22. Oktober 2009 12:29 An: Kirsch, Peter Cc: [log in to unmask] Betreff: Re: [SPM] Contrast error in Flexible Factorial Design Dear Peter, if I get you right you did the following: 24 subjects overall 2 groups 2 conditions for every subject You modeled the subject factor as main effect on 2nd level. Then question is know how you modeled the factors group and condition. One good way to do this would be to model the interaction between group and condition. If you model your desing this way, you get 28 columns. Column 1-24 are the subjects. Column 25 is condition 1 for all subjects of group 1. Column 26 is conditon 2 for all subjects of group 1. Column 27 and 28 are condition 1 and 2 for the subjects of group 2. If you are interested in condition 1 > condition 2 in group 1 your contrast vector would look like [zeros(1,24), 1, -1, 0, 0]. Condition 1 > condition 2 in group 2 would be [zeros(1,24), 0, 0, 1, -1]. These are simple effects, however. If you are interested in the main effect, your contrast vector would be [zeros(1,24), 1, -1, 1, -1]. Hope that helps, BaSti Zitat von Peter Kirsch <[log in to unmask]>: > Dear colleagues, > > I would be very grateful if anybody would have an idea how to solve the > following problem: > > I set up a 3 factorial mixed effects design with 24 subjects using the > flexible factorial design option in SPM5. I have one wihtin-subject factor > (2 levels: condition) and one between-subject factor (2 levels: group). I > modelled the subject effect as the first factor. The design matrix was > estimated but when I try to calculate contrasts, SPM tells me that my > contrasts are invalid. > The first contrast is testing condition 1 > condition 2 and I defined it as > follows: > [zeros(1,24), 1, -1] > Any contrast with less then 25 columns is working but whenever I try to > define a contrast for my experimental factors, I get an error message. > I am quite sure that there is an answer to this in the depth of the archive > but unfortunately I could not find it. > > Thanks in advance for your invaluable support, > > Peter > > -- > Peter Kirsch, PhD > Division for Imaging in Pychiatry > Central Institute of Mental Health > J 5 > D-68159 Mannheim, Germany > >