As far as I know there is not a single contrast that will test that
relationship. The reason being is as follows:
G1>G2 --> 1 -1
G2>G3 --> 1 -1
Now combining these two would produce 1 0 -1 (G1>G3). G2 has no
effect. Because you are looking for difference, the mean of the
contrast has to be 0.
It seems like the better way to ask the question is: where is G1>G2 AND G2>G3.
Use 2 contrasts and then either:
(1) form a conjunction in SPM; or
(2) create 2 thresholded maps, make them into binary images, and then
sum the images. Regions where the value is 2, then G1>G2 AND G2>G3.
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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On Wed, Oct 26, 2011 at 3:28 PM, Shashwath Meda <[log in to unmask]> wrote:
> Hi All - What would be the contrast to identify voxels that show a
> group1>group2>group3 directional effect in a one-way ANOVA design?
>
> --
> with regards,
>
> Shashwath Meda
>
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