Hi. What you have suggested doesn't quite work; it is sensible to demean
EV3, but you can't have this information in a single EV _if_ you are also
telling the "group" setup to split the subjects into two separate
groups...so it looks like you two options:
The first (probably more practical) option is to keep your EVs as you
currently have them, but set all subjects to the same variance group.
The second option is to split EV3 into two EVs, each with half the values
set to zero and the others derived from the age values (with the set of
age values in each EV demeaned separately). However you probably have too
few subjects for this to be a sensible option.
Cheers, Steve.
On Wed, 26 Nov 2003, Rombouts, S.A.R.B. wrote:
> Hi all,
>
>
>
> I have a question about higher-level analyses:
>
> I have 2 groups to compare on fMRI activation, but I suspect that some
> differences between the groups are caused by differences in age.
>
> Therefore, I want to include age as additional covariate (hoping to make
> sure that variance explained by both 'group difference' and 'age' actually
> gets associated with either of these two, not with both).
>
> This is the model I hoped to use (simplified):
>
>
>
> Group EV1 EV2 EV3
>
>
>
> 1 1 0 -5
>
> 1 1 0 -5
>
> 1 1 0 -5
>
> 1 1 0 3
>
> 2 0 1 2
>
> 2 0 1 4
>
> 2 0 1 -1
>
> 2 0 1 7
>
>
>
> , where EV3 is the demeaned age. However, because I have 2 groups, I
> understand that FSL wants me to split EV3 in two EVs, each with only
> non-zeros for one group (that is, after demeaning again, -2 -2 -2 6 0 0 0 0
> and 0 0 0 0 -1 1 -4 4). I guess this would not help me in reaching my goal.
>
>
>
>
> Hopefully anyone has tried this type of analysis too and has some advice how
> to get rid of variance explained by covariates of no interest in a
> two-sample t-test?
>
>
>
> Thanks a lot,
>
> Serge.
>
>
>
>
>
> Serge ARB Rombouts, PhD
>
> Dept Neurology / FMT / Alzheimer Center
>
> VU Medical Center
>
> Amsterdam
>
> The Netherlands
>
> Tel +31 20 4440316
>
>
>
>
Stephen M. Smith DPhil
Associate Director, FMRIB and Analysis Research Coordinator
Oxford University Centre for Functional MRI of the Brain
John Radcliffe 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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