See Jeanette Mumford's web page on centering/demeaning.
cheers,
-MH
On Tue, 2011-12-20 at 14:50 +0000, Heather Collins wrote:
> Dear FSL experts,
>
> I am running TBSS on 2 groups of subjects. The mean ages are not different between the groups. However, when any diffusion metric is plotted over age, the groups show different slopes: one increases with age whereas the other decreases with age. (It looks like an X)
>
> I have combed through the archives and cannot find information about this kind of situation.
>
> 1) Should age be used as a covariate in this situation?
>
> 2) What does it mean for age to be demeaned within each group versus demeaning both groups using the grand mean? Which should I use in this situation?
>
> Thank you in advance for your time and help!
>
> Cheers,
>
> --Heather
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