Hi,
There are a number of ways of resolving this rank deficiency. One way
would be to replace the subject mean EVs with one constant height mean
EV for all of them and one modulated by the covariate, orthogonalised
wrt the constant height one. However you have now no longer modelled
all subject means separately, so this is a less flexible model in
terms of modelling subject mean variability. An alternative, which
sounds a little odd but I think is ok, is to use the original basic
paired design, and enter your new covariate as elements of a
_contrast_ over the subject mean EVs (I guess you'd need to demean all
the covariate values first). This should work ok.
Cheers.
On 24 Oct 2008, at 20:22, Jeske Damoiseaux wrote:
> Hi,
>
> I would like to run a higher level repeated measures analysis on a
> group of
> 8 subjects scanned twice including a covariate (time between 1st and
> 2nd
> scan). I’m interested in the difference between the 1st and 2nd scan
> and the
> effect of the time between the 2 scans. I’m not sure how to set up the
> design. If I use the design for a paired t-test from your example on
> the
> website and add the time difference (demeaned) as an extra EV the
> design
> becomes rank deficient. Is there a way to set up a design combining
> the
> repeated measures with the covariate or do I have to run these
> separately?
> And if I have to run them separately, how do I set up the design for
> the
> covariate? I’ve looked through the archives but couldn’t find a
> suitable answer.
>
> Thanks,
> Jeske
>
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Stephen M. Smith, Professor of Biomedical Engineering
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