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/ -+
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|