Dear list,
I'm puzzled about this: I have a global volume effect in a PET study
despite using global normalization of the data. My design is multi-subject
(n=10), 2 conditions, 1 scan per condition (the scan is an average scan of
two replications of each condition), no covariates. I use Grand Mean
scaling to 50 and 0.8 as the threshold for masking. I tried global
normalization using AnCOVA or proportional scaling: in both cases, SPM96-
and SPM99- analyses, respectively, result in a global decrease (only one
big blob at p=0,001 uncorrected) for one contrast, while there is no effect
for the opposite contrast.
I always assumed that global normalization should make this very case
impossible. Any explanations or suggestions?
Thank you very much
Alex Gamma
University Hospital of Psychiatry
Research Unit
Lenggstr. 31
8029 Zurich
Switzerland
Email: [log in to unmask]
Phone: +41 1 384 26 32
Fax: +41 1 384 33 96
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