Your null hypothesis is: T4-C4=T2-C2
If we make this equal to 0, you get: T4-C4-T2+C2
Create the contrast for T4, C4, T2, and C2.
Now apply the weights from above to each contrast.
1 for T4 and C2 and -1 for C4 and T2
Now add up the 4 contrasts.
For this to work, you need to have specified an interaction term in the model.
The contrast will be: 0 0 0 0 1 -1 -1 1 zeros(1,N) where N is the
number of subjects OR
0 0 0 0 -1 1 1 -1 zeros(1,N) depending in the order of the levels of
each factor.
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 Sat, Jul 27, 2013 at 7:33 PM, Kris <[log in to unmask]> wrote:
> Dear SPM experts,
> Question from a newbie: I have a 2 x 2 within-subject design (task, control) x (2, 4 words), modeled in a flex factorial design using SPM5.
>
> I want to run the following contrast: (task:4 words - control:4 words) > (task:2 words - control:2 words). That is, I want to find increased task-related activity with the respective control conditions subtracted out. How would one go about this?
>
> Would I compute each individual contrast at the 2nd level, save the image files, and then use imcalc to compare them? If so, what operation should be inputted, and what p values/multiple comparison corrections for each contrast are appropriate?
>
> Many thanks,
> Kris
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