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