I really hate to sound like a broken record, but does anyone know if the
effective degrees of freedom are corrected for compound symmetry/dependent
levels of a factor in a full factorial design? Also, can anyone explain to
me why covariances between levels are calculated differently for full and
flexible factorial designs?
Thanks!!
Allison
On Tue, 5 Aug 2008 10:15:34 -0400, Nugent, Allison C. (NIH/NIMH) [E]
<[log in to unmask]> wrote:
>Hi,
> I'm reposting this under a different heading, as the original thread
has gone in a different direction. Basically, I'm confused as to how to
best analyze a PET study.
>
> I have two (unbalanced, N=21 and N=23) groups. Each subject was
scanned (PET) before and after treatment. Initially, I modeled this using a
full factorial, with only group and treatment factors. I specified that the
condition should be a dependent factor. Thus, the resultant covariance
matrix had compound symmetry. It was my understanding that the degrees of
freedom would be adjusted to reflect the dependencies between scans, but the
effective degrees of freedom (SPM.xX.erdf) is equal to (2*(21+23))-4 = 84 -
which is what I would expect for the degrees of freedom if there were no
dependencies. Does that mean that the degrees of freedom *aren't*
corrected, and I must use a flexible factorial with subjects as a factor?
>
> I also tried modeling this with a flexible factorial, incorporating
subject as a factor. I have another issue here. The covariances calculated
between the before and after treatment images are much smaller than those
calculated in the full factorial model, which I found to be very odd, and
I'm hoping someone can explain this to me. (I think this was pointed out in
an earlier post:
http://www.jiscmail.ac.uk/cgi-bin/webadmin?A2=ind0701&L=SPM&P=R47197).
>Which covariances are correct?
>
>Thanks for all your help!
>
>Allison
|