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help > RE: Exchange block and contrast questions
Jan 19, 2017 12:01 AM | Andrew Zalesky
RE: Exchange block and contrast questions
Hi Jeff,
The exchange block vector is simply used to identify the same subject at different time points in a repeated measures design. For example , suppose the rows of your design matrix are ordered such that: S1T1, S2T1, S1T2 S2T2, where S1TI means time point 1 (T1) of subject 1 (S1). In this case, the exchange blocks would be: [1 2 1 2], since the first subject is positioned at rows 1 and 3, and the second subject is positioned at rows 2 and 4.
Alternatively, if the rows were ordered such as S1T1, S1T2, S2T1 S2T2, then the exchange block would be [1 1 2 2].
I'm not sure if I follow your 2nd question about "reverse contrasts". Seems like a complicated contrast. The FSL mailing list has some discussion of the this example.
Andrew
Originally posted by Jeff Peters:
The exchange block vector is simply used to identify the same subject at different time points in a repeated measures design. For example , suppose the rows of your design matrix are ordered such that: S1T1, S2T1, S1T2 S2T2, where S1TI means time point 1 (T1) of subject 1 (S1). In this case, the exchange blocks would be: [1 2 1 2], since the first subject is positioned at rows 1 and 3, and the second subject is positioned at rows 2 and 4.
Alternatively, if the rows were ordered such as S1T1, S1T2, S2T1 S2T2, then the exchange block would be [1 1 2 2].
I'm not sure if I follow your 2nd question about "reverse contrasts". Seems like a complicated contrast. The FSL mailing list has some discussion of the this example.
Andrew
Originally posted by Jeff Peters:
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
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
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Title | Author | Date |
---|---|---|
Jeff Peters | Jan 18, 2017 | |
Andrew Zalesky | Jan 19, 2017 | |
Jeff Peters | Jan 19, 2017 | |