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Hi Anderson, Thanks again for your advice with our analysis. There is one more wrinkle, which I did not mention earlier. To set this up at little, we scanned patients and controls at time 1 and time 2 – roughly 3 months apart – (so we have 4 groups: Patient T1, Patient T2, Control T1, Control T2). There are some subjects who participated at one time point only or time point 2 only, and some who participated both at time 1 and time 2 (in the same study). Clearly, the variance between the same subject at T1 and T2 would be less than two different subjects. However, T1 and T2 scans are always in different analysis groups. How would I take care of this? Would it be through sign flipping, or would it be another approach, since the repeat subjects always fall into different groups? Please let me know if I am not being clear, and thank you again SO MUCH for your help! Sincerely, Alex