Carl,
Assuming that your patients were entered as the first group, then this
contrast would be correct.
Mean-centering will not change the slopes in anyway; however, it does
change the beta of the first two columns. For more details, please
see: http://mumford.fmripower.org/mean_centering/
Also, if you don't want the group means to change after adding the
covariate, then you should mean-center each group separately.
Best Regards, Donald McLaren
=================
D.G. McLaren, Ph.D.
Research Fellow, Department of Neurology, Massachusetts General Hospital and
Harvard Medical School
Postdoctoral Research Fellow, GRECC, Bedford VA
Website: http://www.martinos.org/~mclaren
Office: (773) 406-2464
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On Mon, Nov 26, 2012 at 11:27 AM, Carl Johan Ekman
<[log in to unmask]> wrote:
> Thank you Paul for your suggestion and the very useful papers!
>
> I tried this model by using a Full factorial design with one factor with two levels (patients-controls) and modelling the volume as a covariate with interaction with factor 1. Then I used the contrast: 0 0 1 -1
> I believe this model will calculate in which voxels the slope of the volume vector is steeper in patients than controls.
>
> Is this correct?
>
> If correct, do you have any suggestion on if, or how, the volume vector values should be centered?
>
> Best Regards
>
> Carl Johan Ekman
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