It depends on what you believe about how group membership affects the change of the thickness with age. If you believe that it does not affect it, then use doss. Otherwise use dods. You will lose a dof with dods.

doug

On Wed, 7 Jun 2006, Antao Du wrote:

I want to test the difference of cortical thickness in two groups and age is accounted for as a covariate. Should I use "different offset, same slope" or "different offset, different slope" in GLM model?

Thanks,

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