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Dear Blanca, >Dear SPM users, > >I am carrying out a VBM study. I would like to carry out a factorial analysis >both between and within subjects. Times (time 1 and time 2) would be the within >subjects factors (it is alongitudinal study) and group (group 1 and group 2) >would be the factor between subjects. We want to know whether the volume >decrease is different between the two groups (interaction between two factors). >Does anybody know which statistic model we can use? > I'm not an expert in VBM analysis but I think you can just compare your two groups. For your within condition, subtract T2 vs T1 .. you will obtain a difference (decrease) for each subject. Next, compare Gp1 vs Gp2 and will have the difference between groups on the difference (decrease) of gray matter in each group = interaction :-) Best, cyril -- Pernet Cyril Neuropsychologist, PhD INSERM U455 Pavillon Riser CHU Purpan 31059 Toulouse France Tel: +33(0)561779503 Fax: +33(0)561499524 [log in to unmask]