Dear SPMers,
using VBM we are looking genotype dependent effects in structural scans (spm5).
The genotype can be encoded as a factor with three levels (different group sizes in our case) -
Variances were assumed to be unequal, a set of mean centred covariates (no interactions) is also entered.
Assuming that the group levels are in the first 3 regressors,
how is a T-contrast correctly defined that compares p o o l e d groups 1 and 2 against 3?
(+0.5 +0.5 -1?)
In what way is such a test (if allowed at all) different from pooling the groups 1 and 2 from the start (giving one df more)?
I expected subtle results between the two models, but the differences were larger than just some voxels.
Thanks a lot for any help here,
Philipp
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