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good morning, I have a matrix with 2 groups, 15 subjects per group, same task. there is one task variable and one baseline variable per subject. to look at one group task (first column) minus baseline (second column), I'm putting in a contrast of 1 -1 1 -1 1 -1 ...for 15 subjects no problem for second group 0 0 0 0 0 ... for 15 subjects, then 1 -1 1 -1 1 -1 ...for 15 subjects no problem. now I'd like to look at strictly subtracting the task from group one minus the task from group 2 without doing a double subtraction (without subtracting baseline). 1 0 1 0 1 0....15subjects, -1 0 -1 0 -1 0.... 15 subjects and I'm getting an invalid contrast error. is this an invalid contrast and why? if It's a machine thing, I'm using matlab 6.5, XP, SPM2 thanks you for any ideas. sophie -- Sophie Josee Lafaille, B.Sc.(Hons.), M.Sc. Research Officer II, Speech Fluency Laboratory 1059 Department of Speech-Language Pathology, University of Toronto, 500 University Avenue, Rm 160, Toronto, Ontario, M5G 1V7 Phone: 416-946-8635 Fax:416-978-1596 We don't stop playing because we grow old, we grow old because we stop playing. - Anonymous