[Freesurfer] Group thickness analysis, 2x2x2

Dear Doug et al.,

I have 2 groups, 2 genders, and 2 scanners (A and B). (I also have Age as a covariate). In my FSGD they are listed in this order:

ControlFemaleA ControlFemaleB ControlMaleA ControlMaleB CaseFemaleA CaseFemaleB CaseMaleA CaseMaleB

I used the following contrast to test for a group X scanner interaction: [1 -1 1 -1 -1 1 -1 1 0] And I also looked at a group X scanner X gender interaction: [1 -1 -1 1 -1 1 1 -1 0]

For both, there are no significant results in either hemi. Would this mean that I can instead just have 4 groups (Control/Case Female/Male), or even just 2 groups (Control/Case)? Is that even appropriate?

Thanks, Chris