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At 01:58 PM 12/4/98 -0400, you wrote: | Some precisions please about a paragraph from your answer on | 9/25/98, 18:21 +010. (archived as http://www.mailbase.ac.uk/lists/spm/1998-09/0105.html) | >Firstly, you have to prepare your covariate: Say your scores for | >the first subject are 1b, 1t for pre & post treatment respectively, | >2b & 2r for the second subject, 3b & 3r for the third... Then | >compute thechange in score for each subject, giving covariate: | > 1t-1r, 2t-2r, 3t-3r,... | >Now mean correct this covariate (i.e. work out it's mean and subtractit | >from each element), say the mean is m, then you've got | > 1t-1r-m, 2t-2r-m, 3t-3r-m,... | >Now divide it by two: (1t-1r-m)/2, (2t-2r-m)/2, (3t-3r-m)/2,... | >...and your covariate (assumming you enter your pre-treatment scans first | >for each subject) is | > -(1t-1r-m)/2, +(1t-1r-m)/2, -(2t-2r-m)/2, +(2t-2r-m)/2, -(3t-3r-m)/2,... | | If I understood correctly the design should be as below | pre treatment scores are 1b...........nb | posttreatment score are 1t............nt | change in score are then 1t-1b, 2t-2b,...........nt-nb | mean correct then (1t-1b)-m ..................(nt-nb)-m) | divide by 2 | [(1t-1b)-m]/2 ..........[(nt-nb)-m)]/2 | covariates are if I enter pre-treatment scans first for each subject | - [(1t-1b)-m]/2, + [(1t-1b)-m]/2,.........,- [(nt-nb)-m)]/2, + [(nt-nb)-m)]/2 That's right. I'm afraid I confused b (for baseline) and r (for rest) in my original description, but you've interpreted it correctly. Regards, -andrew +- Dr Andrew Holmes [log in to unmask] | ___ __ __ Wellcome Department of Cognitive Neurology | | ( _)( )( ) Functional Imaging Laboratory, Stats & | | ) _) )( )(__ 12 Queen Square, Systems | | (_) (__)(____) London. WC1N 3BG. England, UK | +------------------------------------- http://www.fil.ion.ucl.ac.uk/ -+ %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%