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 > > > _______________________________________________ > Freesurfer mailing list > Freesurfer@nmr.mgh.harvard.edu <mailto:Freesurfer@nmr.mgh.harvard.edu> > https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer <https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer> -- Douglas N. Greve, Ph.D. MGH-NMR Center greve@nmr.mgh.harvard.edu <mailto:greve@nmr.mgh.harvard.edu> Phone Number: 617-724-2358 <tel:617-724-2358> Fax: 617-726-7422 <tel:617-726-7422> Bugs: surfer.nmr.mgh.harvard.edu/fswiki/BugReporting <http://surfer.nmr.mgh.harvard.edu/fswiki/BugReporting> FileDrop: https://gate.nmr.mgh.harvard.edu/filedrop2 <https://gate.nmr.mgh.harvard.edu/filedrop2> www.nmr.mgh.harvard.edu/facility/filedrop/index.html <http://www.nmr.mgh.harvard.edu/facility/filedrop/index.html> Outgoing: ftp://surfer.nmr.mgh.harvard.edu/transfer/outgoing/flat/greve/ <ftp://surfer.nmr.mgh.harvard.edu/transfer/outgoing/flat/greve/> _______________________________________________ Freesurfer mailing list Freesurfer@nmr.mgh.harvard.edu <mailto:Freesurfer@nmr.mgh.harvard.edu> https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer <https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer> The information in this e-mail is intended only for the person to whom it is addressed. If you believe this e-mail was sent to you in error and the e-mail contains patient information, please contact the Partners Compliance HelpLine at http://www.partners.org/complianceline <http://www.partners.org/complianceline> . If the e-mail was sent to you in error but does not contain patient information, please contact the sender and properly dispose of the e-mail.
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