[Freesurfer] How can I include 3 covariates in my GLM model

Hello Freesurfer's,

I am using QDEC and I have created and FSGD descriptor file with 2 groups (pts and ctrls) and three continuous variables: age, IQ and mean thickness. I have 2 questions:

1. I can only choose 2 covariates at one time? How can I choose all three covariates in one model in my design? 2. I tried group and 2 covariates combinations to check if it runs ok and found that when I select age and IQ, it runs...when I select IQ and mean thickness it runs but it fails when I try running with age and mean thickness in the model? The error in the terminal says matrix not correct, check duplicates..., etc..

What I am doing wrong?

Thanks, Manish Dalwani