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Hi - it sounds like you should be finding this interaction as additional contrasts at the 3rd level, not via a 4th level analysis - does that sound possible? Cheers. On 25 Jul 2007, at 23:54, Stephane Jacobs wrote: > 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 ------------------------------------------------------------------------ --- Stephen M. Smith, Professor of Biomedical Engineering Associate Director, Oxford University FMRIB Centre FMRIB, JR Hospital, Headington, Oxford OX3 9DU, UK +44 (0) 1865 222726 (fax 222717) [log in to unmask] http://www.fmrib.ox.ac.uk/~steve ------------------------------------------------------------------------ ---