Dear Prof. Gaser and CAT12 experts

I have the following queries;

1) For between-group comparison of cortical thickness, is it better to include mean global thickness as a nuisance covariate?

2) If so, how can I obtain mean global thickness for each subject? 

According to the manual, I've tried 'Extract ROI Data' --> 'Extract ROI-based surface values' --> 'ROI analysis', and got several csv files that contained information regarding the cortical thickness for each parcellated cortex (i.e., ROI_catROIs_aparc_a2009s_thickness.csv).

However, I cannot find mean global thickness value for each subject.

Thank you in advance for your comments.

LK