Thank you very much for your quick reply.

It's true that I want to test specific hypothesis. This is an aging study and I am using diffusion data to build structural connectivity. So, I want to test if structural connectivity will decrease with time but increases could also happen, so I want to test both hypothesis: time1 > time2 > time3; time1< time2 < time3. How can I do this?
Another thing, the time between acquisitions in the 3 timepoints is not the same for all subjects. From time1 to time2 it varies between 16 to 31 months and from time 2 to time3 between 25 to 55 months. Should I include this in the model as cofounds? If true, how can I do this?


Thanks in advance!
Best regards,
Ana