Hi Dost,
I just wanted to note that (as far as I can tell) the same issue
occurs even when a covariate is available for both groups, using the
recommended FSL approach of demeaning the covariate separately with
respect to each group and padding with zeros for the other group.
This method does not address the question of whether/how a group
difference in values of the covariate might affect the observed
magnitude of the group difference in means.
Dan
On 3/13/07, Dost Ongur <[log in to unmask]> wrote:
> Hello all,
> I have scanned two groups of subjects: patients and controls. For the
> patient group, I have a symptom scale that I believe may be relevant, but
> this scale does not apply to controls.
> I would like to include this symptom scale as a covariate to remove any
> contribution it makes to the group difference. Is it acceptable to create
> a third EV and use demeaned numbers for the patients and zeros for
> controls? It seems that approach assumes that the controls have the
> patient mean.
> Any thoughts would be appreciated.
> Thanks,
> Dost
>
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