[Freesurfer] Statistical analysis

Dear FS team,

Thank you very much for your help. Just a few more questions.

I want to perform a statistical analysis between two groups:controls and patients, using age and IQ as covariates. First I have to test if there is an interaction between group and covariate using DODS.

My questions are:

Q1) Do I have to test age and IQ separately, or together?

Q2) In case that there is not interaction in any of the covariates, can I use the same FSGD file but do I have tu run it as DOSS? How it could be done? I would appreciate if you could give me some hints or examples about the matrices and contrast files that I must use.

Thank you, All the best,

Laura