Dear FreeSurfer experts,

Hello, I started to use FreeSurfer recently. I want to do group analysis with QDEC in order to compare 2 groups (control group and patient group), regressing out age and gender.

When I include 2 discrete factors (diagnosis and gender) and 1 continuous factor (age), and run QDEC, there appear a question of "Does the average thickness, accounting for Gender, differ between Controls and Patients?", but there NOT appear a question of "Does the average thickness, accounting for Gender *AND AGE*, differ between Controls and Patients?". So, it seems that age cannot be regressed out in this way.

However, when I include 1 discrete factor (gender only) and 2 continuous factors (age and also diagnosis *dummy parameterized: Controls = 1, Patients = 0*), and run QDEC, there appear a question of "Does the correlation between thickness and Diagnois, accounting for Age and Gender, differ from zero?", and the result seems OK.

My question is; 1) Is it right to do the (above-mentioned) latter way to compare difference of thickness between the 2 groups, regressing out age and gender? 2) Why couldn't age regressed out in the former way? 3) In this way, generally is it OK if I enter simple "discrete" factor as a continuous factor in QDEC (for example, gender:male=1, female=0)?

This time I want to ask, since I couldn't find an answer in the past archives. Thanks a lot,

Manabu Kubota kubotajp@kuhp.kyoto-u.ac.jp Graduate School of Medicine, Kyoto University