Hi Andrew,

Thanks very much for your reply. 

Just to be clearer about my design, I'm looking at changes in edge weight between two age groups. Some subjects are imaged in both age groups, most are not.

Since sex is unchanging over time, and is modelled by the within subject columns, I can remove the sex column from the design matrix. This leaves age, age*sex, and intra-subject dummy variables. To then test the main effect of age group I would use [1 0 0 ...]. Have I got this right?

Thanks for your help!