Dear All,
I have a question that has recently come up in a discussion. Background
is, we are doing a VBM study of three groups, say A/B/C. We want to
perform an analysis on "modulated" GM maps, as derived by a DARTEL
procedure. As such, it is usually recommended to include a global
covariate, either total GM volume or total intracranial volume, as a
covariate. Our groups, however, due to the nature of the underlying
condition, have different globals, i.e., group A has higher globals than
group B and/or C.
I understand that including the globals will change the interpretation
of the resulting group differences. It may also, due to the group
difference in the globals, "take away" some effects that may exist
between the groups because the variance may be shared.
One idea that then came up was whether it is possible to use an
orthogonalization on total GM / TIV, w.r.t. group status. This seemed
like a worthwhile idea at the time as the aim was to not explain group
differences by this orthogonalized variable that are already explained
by group. We tried it and the effect was substantial, to say the least.
I am not sure, though, that this is a good (or even valid) idea from a
statistical point of view. For one, I have seen several mails in the
archives that mention that othogonalization of a nuisance variable is
not a good idea. One could of course argue that it is not really a
nuisance variable, it "only" changes the interpretation of the results
by (interpreted in a very naive way) rescaling. But then again, I could
be totally wrong and not see the forest for the trees. So as always,
your insights into this matter are much appreciated.
Cheers
Marko
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Prof. Dr. med. Marko Wilke
Facharzt für Kinder- und Jugendmedizin
Leiter, Experimentelle Pädiatrische Neurobildgebung
Universitäts-Kinderklinik
Abt. III (Neuropädiatrie)
Marko Wilke, MD, PhD
Pediatrician
Head, Experimental Pediatric Neuroimaging
University Children's Hospital
Dept. III (Pediatric Neurology)
Hoppe-Seyler-Str. 1
D - 72076 Tübingen, Germany
Tel. +49 7071 29-83416
Fax +49 7071 29-5473
[log in to unmask]
http://www.medizin.uni-tuebingen.de/kinder/epn/
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