Dear All,
at the risk of (once again) exhibiting my statistical ignorance, I
wanted to ask for feedback re: an effect I see when doing F-tests. I am
comparing sessions with different numbers of covariates and compute an
omnibus F-test in SPM (for the covariates only) and sum the resulting
F-values. As expected, I see that there seems to be an optimum number of
covariates as the sum of F-values first increases, then decreases when
adding more covariates. I expected this to be a function of the degrees
of freedom but I cannot seem to find the piece of code where these are
taken into account. Any input is appreciated.
Cheers,
Marko
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PD 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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