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Dear Will Penny, John Ashburner, and other SPM2 experts



This is a question that relates both to ANOVA/ANCOVA models and VBM.



I'm running a VBM study to delineate local differences in GM volume between
a group of dyslexics and a control group.



For the statistical analysis I’m using the “basic model” in spm2.

I have a question about non-sphericity correction option that gives me
different estimates (SPM.xVi.h) for the two groups depending on the model I
use.

First, I applied ONE WAY ANOVA with non sphericity correction option and
global calculation (mean voxel value (within per image full mean/8 mask )
to obtain an estimate of the subject-specific over-all GM volume. In this
analysis the non-sphericity values are similar for the two groups
(SPM.xVi.h values for dyslexics are 0.9772 and for the control group
1.0228).

Then I used the GM volume values obtained in the first analysis
(SPM.xGX.rg) as covariate in the ANCOVA with non sphericity correction and
obtained SPM.xVi.h values for dyslexics of 1.2857 and for the control group
0.7052.



I checked the mask images in the ANOVA and ANCOVA analyses: they are
identical, indicating that the same brain regions are considered.



1. I assume that my design matrix in the ANCOVA and ANOVA are identical
(since the GM-volumes are modelled in both cases). Is this true?



2. In that case, why are the SPM.xVi.h values so different for the two
models?

Could it be that the sphericity correction is more effective (the hyper-
parameters differ more between groups) for the ANCOVA rather than for the
ANOVA?



3. In a VBM analysis, is it plausible that the variability in GM volume is
so different between groups? Have other people observed similar effects?



Thanks in advance,

Deny Menghini