Dear Kewei,
> We have 3 groups of subjects: A B C. Group A has 7 subjects, 11 in B
> and 14 in C. Each subject in any one of the three groups has two
> longitudial FDG-PET scans (roughly two years apart). Scan 1 is the
> baseline scan and follow-up scan is scan2.
>
> We are interested in comparing longitudinal glucose uptake reduction
> (increase) within each group, and compare the reduction between any two
> group (especially between B and C).
>
> Using multi-study, different conditions and proportional scaling design
> in spm96, the glucose uptake reduction in group A, B, and C were tested
> respectively using
> 1 -1 0 0 0 0 %for group A
> 0 0 1 -1 0 0 %for B
> 0 0 0 0 1 -1 %for C
>
> Cross group comparison was made, for example, as 0 0 1 -1 -1 1 (group B
> glucose uptake reduction is > than the one for C) etc.
>
> As a post-SPM analysis, we extracted the voxel values (spm-normalized
> and corrected) for the two scans of each subject from various
> locations. (i.e, XA(:,i) for XYZ(:,i) location)
> What we found is that, for example with group B, for all the locations
> that we have examined so far, highest p1 is always paired with the
> lowest p2, and lowest p1 is always paired with the highest p2. The
> pairs in the middle follow a similiar trends.
This is simply a reflection of the fact that the subject effect has
been removed as a confound and therefore each subject's effects reduce
positive and negative deviations from the grand mean. There is nothing
to worry about (I hope).
With very best wishes - Karl
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