Main Effect of Group:
[1 -1 0 0 0 0 0; 0 1 -1 0 0 0 0]
Individual Group Differences:
1 -1
-1 1
etc.
Your contrasts for group differences will work if you want to test
whether 1 group differs from the average of the other 2 groups.
Interaction Effect:
[0 0 0 1 -1 0 0; 0 0 0 0 1 -1 0]
Comparing the individual group slope differences can be done with your
proposed contrasts for the interaction.
On Wednesday, October 20, 2010, Angie Morales <[log in to unmask]> wrote:
> Hi Everyone,
>
> I'm trying to figure out how set up the design matrix and subsequent contrasts for an interaction between a continuous variable and a factor with three levels (group) at the group level (VBM analysis).
>
> Using the full factorial template (SPM5) I set up my design matrix as follows by entering 1 factor with 3 levels, one covariate (with the specification that I was interested in the interaction with the factor) and one covariate no interaction.
>
> Group1 Group2 Group3 Cov1 Cov2 Cov3 NuisCov
> 1 0 0 8 0 0 25
> 1 0 0 10 0 0 18
> 0 1 0 0 11 0 17
> 0 1 0 0 9 0 30
> 0 0 1 0 0 12 21
> 0 0 1 0 0 14 27
>
> To look for differences in mean gray matter volume between groups, I used the following contrast:
>
> 2/3 -1/3 -1/3
> -1/3 2/3 -1/3
> -1/3 -1/3 2/3
>
> Is this correct?
>
> Which contrast could I use as an omnibus test of the group*cov interaction. Would the subsequent tests be set up as follows:
>
> 0 0 0 1 -1 0 0
> 0 0 0 -1 1 0 0
> 0 0 0 0 1 -1 0
> ...etc.?
>
> Any help would be greatly appreciated. Thanks!
>
--
Best Regards, Donald McLaren
=================
D.G. McLaren, Ph.D.
Postdoctoral Research Fellow, GRECC, Bedford VA
Research Fellow, Department of Neurology, Massachusetts General Hospital and
Harvard Medical School
Office: (773) 406-2464
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