> nobody?
It's a stats question, so I left it for the guys who know about stats, whereas
they saw it as a morphometry question, so left it for me. As a rule, if you
want an authoritative answer about stats, it is better not to mention that
you are doing VBM.
> I have a optimized and modulated vbm study with cases and controls, I want
> to compare for gm reduction in cases.
> I have choosen PET model Ancova (compare two populations) and I have two
> nuisances, global brain volume and age.
> How to set contrast for controls>cases eliminating effects due two global
> volume and age? 1 -1 0 1 1 or 1 -1 0 0 0?
I'm not 100% sure what this design matrix looks like, but I suspect that the
first two columns indicate which subjects belong to the two groups. In taht
case, contrasts of [1 -1 0 0 0] and [-1 1 0 0 0] can be used to answer this
question..
> On the other hand is it possible possible with this model to see if any
> correlation between a found decrease in gm and the two covariates or
> others?
Do you mean look for an interraction between group membership and the
contrasts? If so, then you'll need a slightly different design matrix, which
models the covariate seperately for each group. I'm not sure what options in
the stats UI will give you this.
Best regards,
-John
|