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 > <gr...@nmr.mgh.harvard.edu <mailto:gr...@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 > gr...@nmr.mgh.harvard.edu <mailto:gr...@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. > > > > > _______________________________________________ > Freesurfer mailing list > Freesurfer@nmr.mgh.harvard.edu > https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer -- Douglas N. Greve, Ph.D. MGH-NMR Center gr...@nmr.mgh.harvard.edu Phone Number: 617-724-2358 Fax: 617-726-7422 Bugs: surfer.nmr.mgh.harvard.edu/fswiki/BugReporting FileDrop: https://gate.nmr.mgh.harvard.edu/filedrop2 www.nmr.mgh.harvard.edu/facility/filedrop/index.html Outgoing: ftp://surfer.nmr.mgh.harvard.edu/transfer/outgoing/flat/greve/ _______________________________________________ Freesurfer mailing list Freesurfer@nmr.mgh.harvard.edu https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer
