Here's an odd thing.
We're analysing PET group differences (depressed vs control) with SPM96.
Problem is, when we do the global normalisation, it seems to 'compress' the
values for the control group into a much smaller range so that the column
in the design matrix representing these subjects is almost invisible. The
values given are tiny compared to the depressed patients. If we allow the
analysis to run its course, it is clear that only the depressed subjects
contribute to the resultant SPM(Z)s. The global CBFs are indeed
consistently lower for the controls but we have no problems when we analyse
the groups separately. I've tried subject-specific, group-specific and
simple ANCOVAs.
Has anyone come across this? Any ideas?
We'd be most grateful for any advice
Yours faithfully,
Paul Fletcher
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