Dear freesurfer experts,

I'm using Qdec and I want to control for age (a continuous variable) and sex (f,m or 1,0).

Do I need to center age (i.e., subtract the score from its mean) before I select it as a nuisance factor?
Do I need to center sex?

I found that selecting age_centered as a nuisance factor vs. selecting age_uncentered as a nuisance factor led to very different results… not sure what is the correct way to go. 
In brief, if I want to control for age and sex, which of the following is the correct way to go?
  1. Center neither age nor sex (f,m). Then choose age_uncentered as a nuisance factor, but sex_uncentered as a discrete/fixed factor.
  2. Center age but not sex (f,m). Then choose age_centered as a nuisance factor, but sex_uncentered as a discrete/fixed factor.
  3. Center age but not sex (1,0). Then choose both age_centered and sex_uncentered as nuisance factors.
  4. Center age and sex (1,0). Then choose both age_centered and sex_centered as nuisance factors.
Thanks in advance!
Daniel
-- 
Yung-Jui "Daniel" Yang, PhD
Postdoctoral Researcher
Yale Child Study Center
New Haven, CT
(203) 737-5454