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Dear SPM-experts, I would like to compare 3 different groups (A, B & C), each comprising a different number of subjects (nA, nB and nC). Setting up a one-way ANOVA (1 factor with 3 levels) allows for an overall comparison (f-test) and post-hoc t-tests between groups (t-tests). However, now I need to compare groups (A & B) vs C and I am not sure about the correct t-contrast: 1) is it simply [0.5 0.5 -1]? 2) or do groups A and B require a weighting with the corresponding sample size in each group, ie. [nA/(nA+nB) nB/(nA+nB) -1]? Both would give me a balanced contrast, but I am not sure whether the number of subjects per group are already accounted for (-> contrast 1) or if one needs to consider this in the contrast setting manually (-> contrast 2)? I already checked the Glascher and Gitelman-tutorial as well as the SPM archives, however, I could not find an answer to this (rather simple) issue. Thank you in advance for your help and kind regards, Andreas. -- Andreas Hahn, MSc. Functional Neuroimaging Group - PET & fMRI Department of Psychiatry and Psychotherapy Medical University of Vienna, Austria Phone: +43 1 40400 3825 Email: [log in to unmask] Web: http://www.meduniwien.ac.at/neuroimaging/