Dear Guillaume,
Thank you for your reply. Actually, I talked to a colleague who gave me some advice how to properly enter the contrasts:
Main effect of time: 1 -1 1 -1
Interaction of group by time: 1 -1 -1 1
...
When I saw it right, Donald MacLaren stated various times that assessing the group effect using such a design does not give valid estimates... However, we are more interested in the interaction of group by time etc. so that this should not be a problem.
Finally, I wondered if the results change if I add a main effect for the group factor to the design?
Best regards,
Martin
PS: Using your MATLAB code gives the following visualization:
>> load SPM
>> X=SPM.xX.X;X(X&X~=1)=2;c=' |~';
>> c(X+1)
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| ~ ~ |
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| ~ ~ |
| ~ ~ |
| ~ ~ |
| ~ ~ |
| ~ ~ |
|~ ~ |
| ~ ~ |
|~ ~ |
with columns: g1t1, g1t2, g2t1, g2t2, TIV, Site, Age, Subject1, Subject2 ... Subject 15
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