Demeaning age separately within each group would be a very atypical
analysis. Jeanette's page has a paragraph on why this is generally a
no-no.
cheers,
-MH
On Tue, 2011-12-20 at 12:49 -0500, Heather R. Collins wrote:
> Thanks, Yingying.
>
> The two groups are controls and patients and we are interested in
> diffusion metrics associated with the condition and not due to
> developmental changes (age).
>
> What does demeaning age within groups (for controls and patients
> separately) tell us and what would demeaning all subjects (patients
> and controls) with the grand mean tell us?
>
> Many thanks,
>
> --Heather
>
> On Tue, Dec 20, 2011 at 12:00 PM, Yingying Wang
> <[log in to unmask]> wrote:
> Hi Heather,
>
> According to your message, you might want to do the two group
> separately. Also it depends on your hypothesis.
>
> For example: two group: controls and patients
> What's your interest?
>
> 1) Use age as covarite (nuisance variable) if you're not
> interested in developmental changes.
>
> 2) http://mumford.fmripower.org/mean_centering/
>
> Best, Yingying
>
> >>> Heather Collins <[log in to unmask]> 12/20/2011 9:50 AM
> >>>
> 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 covarite 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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