[Freesurfer] Comparing between a group of subject

Hi Freesurferes,

Sorry for my simple and rookie question, I'm a new user and couldn't find my answer after a bit googling.

I have a group of subjects and want to compare their thickness with a baseline (average of my subjects).

I thought to use the mri_avarage to make the baseline (Average of all of my subjects), but don't know how to compare them with the baseline after making the baseline.

Any help would be highly appreciated

Thanks,

Aman

Ps. I have looked into GLM and longitudinal processing tutorials; GLM doesn't work for me as I don't have any variable to regress against; longitudinal processing is also for comparing different time shots of the same subject (my case are 40 different subjects).