Dear Andrew,

I have a within-subject design with 21 subjects and 3 task conditions A, B, and C. Using the FSLwiki (triple t-test section), I was able to create the design matrix (would you mind taking a quick look to see whether it looks fine). Before running the analysis, I have a couple of questions to which I would be grateful for answers:

Contrast: I have the following contrasts: A-B=[2 1 0...], B-C=[-1 1 0...], A-C=[1 2 0...]. I am also interested in the reversed contrasts (i.e., B-A, C-B, and C-A). Going by what I think is the logics behind contrast setup, I came up with: B-A=[-1 -2 0...], C-B=[-1 1 0...], C-A=[-1 -2 0...]. The C-B contrast seems incorrect as it is the same as B-C. Would you kindly point out where I may have gone wrong here?

Exchange block: from the manual, I see that the exchange block is required for repeated measures within-subject designs, which is what I have here. However, I am confused as to what input goes into the vector. Since I have 21 subjects, the vector will have 21 rows (though a search throughout the forum doesn't always show that users have stuck to this principle). Am I missing something? To really address my ignorance, could you please explain the reason for constraining the permutations?

Best,

Jeff