Dear FreeSurfer experts,

I am performing a DODS analysis with QDEC to compare cortical thickness of patients and controls and I have some questions regarding the nuisance factors age and global cortical thickness.

1.- Should I demean both variables or only the age?

2.- Which of the following options do you think that would be better?

- To include the global mean thickness of the left hemisphere for the QDEC analysis of the left hemisphere and the right global mean thickness for the QDEC analysis of the right hemisphere.

- To include mean cortical thickness calculated like that: bh.thickness = (lh.thickness*lh.surfarea + rh.thickness*rh.surfarea) / (lh.surfarea + rh.surfarea)

If you think that this is the best option, which surfarea should I use? Would it be correct to use the WhiteSurfArea?

Thank you for your help, Yolanda