Excerpts from SPM-help#: 13-Sep-100 comparing line fit across m.. by
Kalina [log in to unmask]
> for the first model:
> Sum_ResMS_1 = (ResMS for Subj No1) + (ResMS for Subj No2) + ...
>
> and for the second model:
> Sum_ResMS_2 = (ResMS for Subj No1) + (ResMS for Subj No2) + ...
>
> and then perform an F-test,
>
> F = Sum_ResMS_1 / Sum_ResMS_2,
>
> with df1 = df2 = the sum of the error df from the subjects' analyses
Kalina,
The immediate problem that comes to mind is that the F test assumes
that the numerator and denominator are independent... something that
is satisfied when you are looking at nested models, but not here.
The F images still have the qualitative interpretation, that of relative
magnitude of variance explained, but the p-values and thresholds won't
be right.
It's an excellent question, because this is a perfect example of an
interesting, but non-nested model selection problem. But I'm afraid I
don't have any solutions off hand.
-Tom
-- Thomas Nichols -------------------- Department of Biostatistics
http://www.sph.umich.edu/~nichols University of Michigan
[log in to unmask] 1420 Washington Blvd
-------------------------------------- Ann Arbor, MI 48109-2029
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