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
>> ---------------------------------------------------------------------
>> ------
>>
>>
>>
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---
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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