I notice you wrote [ones(1,21)/21 ones(1,23)/23]. Did you mean for
both to be positive or was that just a typo? This might explain the
problem.
Darren
On Aug 1, 2008, at 3:09 PM, Allison Nugent <[log in to unmask]> wrote:
> Hello,
> I'm hoping someone can help me with quandry - I'm trying to figure
> out
> the best way to analyze a study.
> I have a PET study, with 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 modelling this with a flexible factorial, incorporating
> subject as a factor. I have two issues here. First, the covariances
> calculated between the before and after treatment images are much
> smaller
> than what was 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?
> Also, I can't seem to enter many of the contrasts I'd like. For
> example, for the group contrast, since I have 21 in group 1 and 23
> in group
> 2, the first 44 elements in the contrast would be:
> [ones(1,21)/21 ones(1,23)/23]
> (ala Jan Glascher's very helpful guide to contrasts)
> However, I get an error - because when matlab evaluates all those 1/21
> entries minus all the 1/23 entries, it produces something not quite
> zero -
> because of rounding errrors - since the contrast elements add up to
> 1E-15
> instead of 0, I'm told the contrast in unestimable. Does anyone
> know of a
> way around this?
>
> Thanks for all your help!
>
> Allison
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