If there was not an interaction between group and ICV, I would say to
use DOSS where demeaning does not make a difference. Since there is an
interaction, you have to use DODS, but the interpretation is no longer
easy. If you do not demean, then you are implictly testing for a
difference at ICV=0 (this is what "regressing out" means). If you
demean, you are implicitly testing at ICV=MeanICV. The problem is that
the results can change depending on the ICV you test it at.


On 08/29/2016 02:53 PM, Corinna Bauer wrote:
> Sorry for not specifying. Yes, we are looking at difference in volume
> between the two groups using DODS. What is the most statistically
> sound... raw values, demeaning across the entire sample, or demeaning
> with groups separately? FYI, I exported the ICV values to SAS and
> there is a significant different between the two groups for ICV.
>
>
>
> On Mon, Aug 29, 2016 at 2:48 PM, Douglas N Greve
> <greve@nmr.mgh.harvard.edu <mailto:greve@nmr.mgh.harvard.edu>> wrote:
>
>     not sure what you are testing, but if you are, eg, looking at the
>     difference between two groups regressing out ICV, then it can make a
>     huge difference
>
>
>     On 08/29/2016 12:36 PM, Corinna Bauer wrote:
>     > Hello all,
>     >
>     > I am wondering why results would change completely for between group
>     > volume analysis when using demeaned/mean-centered ICV compared to
>     > using ICV as a covariate in mri_glmfit? See the figures below.
>     >
>     > Does demeaning the covariate inherently change the results
>     comapred to
>     > the raw covariate values?
>     >
>     > Thanks
>     >
>     > Corinna
>     >
>     >
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