Hi Steve,
Thanks for your help - and patience! That seems so obvious now...
Sorry for the stupid question, hopefully it'll keep somebody else to ask it in the future! =:-)
Best,
Stephane
On Mon, 30 Jul 2007 09:26:07 +0100, Steve Smith <[log in to unmask]> wrote:
> Hi - I'm pretty sure you can do this at the third level. If your 3rd
> level EVs are:
>
> >>> Al > bl, Ar > bl, Bl > bl and Br > bl
>
> Then do ask
>
> >>> (Al>Bl) > (Ar>Br)
>
> At the third level you just want the contrast:
>
> [ 1 0 -1 0 ] - [ 0 1 0 -1 ] = [ 1 -1 -1 1 ]
>
> which is a typical interaction contrast. Note that you may want to
> use contrast masking to ensure that (e.g.) Al>Bl and Ar>Br (etc) to
> help the interpretation of the interaction.
>
> Cheers, Steve.
>
>
>
> On 26 Jul 2007, at 18:16, Stephane Jacobs wrote:
>
> > Hi Steve,
> >
> > Well, I don't think it is possible with the way my analysis has
> > been run
> > up to the 3rd level. If I take the example of the interaction
> > (Al>Ar) >
> > (Bl-Br), I get both contrasts Al>Ar and Bl>Br only at the output of
> > the
> > 3rd level analysis, so I don't see how I could contrast them one
> > against
> > another within the same analysis...
> > Should I set the differential contrasts (e.g. Al>Ar, etc...) as
> > soon as
> > the 2nd level, for each subject? Then I guess I could set my
> > interaction
> > contrasts at the 3rd level...
> > Do you see any reason why this should fail at the 4th level?
> >
> > Thanks again for your help,
> >
> > Stephane
> >
> >
> > Steve Smith wrote:
> >> 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
> >> ---------------------------------------------------------------------
> >> ------
> >>
> >>
> >>
>
>
> ------------------------------------------------------------------------
> ---
> 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
> ------------------------------------------------------------------------
> ---
>
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