Hi there,
I'm running an fMRI experiment that has 3 conditions (A B C), and 2 groups (patients, controls). I've already run different analyses that tests for the different pairwise interactions and simple main effects separately, but now I'd like to include everything in the same model (I'd like to test for 6 interactions).
In a previous post covering this topic ("randomise interaction F-test"), it looks as though I should approach this using an F test.
In the higher level group analysis, I have my .mat file setup as follows (assuming 3 subjects per group, for the sake of simplicity):
1 0 0 0 0 0 (control s1, condition a)
1 0 0 0 0 0 (control s2, condition a)
1 0 0 0 0 0 (control s3, condition a)
0 1 0 0 0 0 (patient s1, condition a)
0 1 0 0 0 0 (patient s2, condition a)
0 1 0 0 0 0 (patient s3, condition a)
0 0 1 0 0 0 (control s1, condition b)
0 0 1 0 0 0 (control s2, condition b)
0 0 1 0 0 0 (control s3, condition b)
0 0 0 1 0 0 (patient s1, condition b)
0 0 0 1 0 0 (patient s2, condition b)
0 0 0 1 0 0 (patient s3, condition b)
0 0 0 0 1 0 (control s1, condition c)
0 0 0 0 1 0 (control s2, condition c)
0 0 0 0 1 0 (control s3, condition c)
0 0 0 0 0 1 (patient s1, condition c)
0 0 0 0 0 1 (patient s2, condition c)
0 0 0 0 0 1 (patient s3, condition c)
And my contrasts are set up as follows:
1 -1 1 -1 1 -1 (to test the main effect of group)
-1 -1 1 1 0 0 (to test the main effect of condition)
-1 -1 0 0 1 1 (to test the main effect of condition)
1 -1 -1 1 0 0 (to test the group by condition interaction)
1 -1 0 0 -1 1 (to test the group by condition interaction)
Finally, my f tests are set up as the following:
1 0 0
0 1 0
0 1 0
0 0 1
0 0 1
... where column 1 tests for group, column 2 tests for the main effect of condition and column 3 tests for the group by condition interaction
My question is: does the above correctly address the interactions?
As a side-note/bit of background, I had each subject complete three separate runs of the experiement. To concatenate these, I used the three level approach wherein statistical analyses were carried out at 3 levels: a single run intra-subject level, a multiple-run intra-subject level, and a higher-level intra- and inter-group level. So, each row in the .mat file above represents a cope image for a given subject during a given condition.
Thanks so much in advance for the help,
Lara
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