With two groups the F-contrast should be:

[1 -1 -1 1]

With three groups, the F-contrast should be:

[1 -1 - 1 1 0 0

0 0 1 -1 -1 1]

This assumes the order of columns is: G1S1, G1S2, G2S1, G2S2, G3S1, G3S2

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 Thu, Jul 10, 2014 at 12:32 PM, MRI Study <[log in to unmask]> wrote:

Dear spm-list,

I have built a flexible factorial design for two groups with two sessions (G1S1, G1S2, G2S1, G2S2) and fiven the following f-contrast weights for the interaction Group x Session:

[1 -1 -1 1
-1 1 1 -1]

Now I would like to add a third group to the design, so it would be G1S1, G1S2, G2S1, G2S2, G3S1, G3S2. Im leaving out the main effects on purpose, so I would just like to model the interaction.

I'm not sure how to define the contrast weights though. Could somebody help me? Thanks!