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Dear SPM-ers, I'm a little confused by the results of my SPM99 analyses. I hope someone can tell me in simple language what the multi-group analysis actually does. I have two groups of subjects (patients and controls), with four H2O-PET scans for each subject, one scan per condition (ABCD). I did a compare-population-means (one scan/condition)test pts (scan A) vs controls (scan A), and also pts (scan D) vs controls (scan D)(contrast 1 -1 (pts-controls) and contrast -1 1(controls-pts)). This revealed some regions in which pts showed more activation than controls and some regions in which pts showed less activation than controls. Then I performed a multi-group analysis pts(A-D) vs controls (A-D)(at least I think I did that by using the following contrasts:), contrast: 1 0 0 -1 - 1 0 0 1 (pts-controls?) and -1 0 0 1 1 0 0 -1 (controls-pts?). I expected to find more or less the same regions more activated in pts than in controls, but I found exactly the opposite. The areas that were more activated in pts vs controls in A and D separately, were the areas that were less activated in pts vs controls in the multi-group analysis and vice versa. I hope someone can tell me where my error of thought (I think it must be something like that) occurs. I hope it's not too stupid. Many thanks already, Marjolein Boshuisen