________________________
Leighton BARNDEN PhD
Medical Scientist
Department Nuclear Medicine
The Queen Elizabeth Hospital
Woodville, SA 5011
Australia
phone +618 8222 6438 fax +618 8222 6038
Thanks Tom,
Quick follow-up: Centering both covariates to the overall mean
should de-mean them - right?
The SD/mean I get still seem (about 4 times) too big. Has anyone else
done this and got reasonable results?
Leighton
> > I would like to characterise the intersubject noise (variance after
> > adjusting out effects) in a population of SPECT scans. I
> have computed the
> > coefficient of variation (SD/mean) image using
> SQRT(ResMS)/beta_0003 (there
> > were 2 covariates in the study).
> > Is this a valid approach?
>
> It depends on the interpretation of beta_0003, the intercept. If all
> of the covariates are mean zero, then beta_0003 is precisely the mean
> of the SPECT data at each voxel. If the covariates aren't de-meaned,
> then beta_0003 is... the intercept, the fitted response when all
> covarariates are zero.
>
>
> > The numbers in grey matter seem a bit big - around 0.6
> > Are the units for the values in SQRT(ResMS) and beta_003
> the same or is some
> > kind of scale factor involved?
>
> When the covariates are de-meaned it should be exactly the coefficient
> of variation, no scaling needed.
>
> -Tom
>
>
> -- Thomas Nichols -------------------- Department of
> Biostatistics
> http://www.sph.umich.edu/~nichols University of Michigan
> [log in to unmask] 1420 Washington Heights
> -------------------------------------- Ann Arbor, MI 48109-2029
>
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