Hello,
I'm trying to set up interaction contrasts in a 4th level-analysis. I have 2
conditions (A and B) x 2 sides (l and r) design, and I'd like to analyze the
interaction between the factors SIDE and CONDITION.
The second level analysis is run for each subject separately to model
between run variance, and outputs each of the 4 conditions vs. resting
baseline (bl): Al > bl, Ar > bl, Bl > bl and Br > bl.
The third level analysis models between-subjects variance, and is basically
a "quadrupled" T-test, computing paired-comparisons between my 4 conditions:
Al > Bl, Ar > Br, Al>Ar, Bl > Br, and the reverse contrasts.
Now, I want to take these copes from the 3rd level analysis to test for
interactions between SIDE and CONDITION. To this end, I set up the following
contrasts in my 4th level analysis:
(Al>Bl) > (Ar>Br) to test the difference between both sides for (A > B)
(Al>Ar) > (Bl>Br) to test the difference between both conditions for (l > r)
etc... I have a total of 8 contrasts set, using 8 EVs coming from the 3rd
level analysis.
I guess (hope!) all this makes sense so far, but FEAT fails running the
analysis when doing higher-level stats:
An exception has been thrown
Singular design. Number of EVs > number of time points. Trace:
Gsmanager::ols; Gsmanager::run.
I can't figure out why this is happening... I understand it suggests I don't
have enough data to run these contrasts, but it surprises me as I have 20
subjects in my group, and quite a few trials per condition. This being said,
I'm not quite sure of what the number of time points represents for such a
high-level analysis...
Any help would be most appreciated!
Best regards,
Stephane
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