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Dear Freesurfer experts,
May I ask how to understand the contrast of interaction between covariates in the example https://surfer.nmr.mgh.harvard.edu/fswiki/Fsgdf2G2V? The interaction of group x age x weight is:
[0 0 1 -1 0 0 0 0 0 0 1 -1]
Why the interaction between group and weight should be put in the second row (what does each row mean in the F-contrast matrix)?
If there are three levels in the factor one (three groups), I guess the interaction of group x age x weight should be:
[0 0 0 1 -1 0 0 0 0 0 0 0 0 1 -1 0 0 0 0 0 0 0 0 0 1 -1 0 0 0 0 0 0 0 0 1 -1]
Is this correct? If not how should I set this contrast?
In addition, if I would like to know the interaction between age and weight without group, how should I set the contrast then?
Looking forward to your kind suggestions.
Thanks and best wishes,
Lizhi