You definitely do not want to demean within group. Having said that, no demeaning method is more "sound" than the other. The problem is in the interpretation. Imagaine you have a plot of volume vs ICV for each group. When you have an interaction, the best-fit lines will intersect at some ICV. If you test at that intersection, then you will get no difference. If you test on one side of the intersection, you will get a difference of some sign. If you test on the other side, you will get a difference of the opposite sign. So your results depend on what ICV you test at.
On 08/29/2016 03:43 PM, Corinna Bauer wrote: > Yes, I agree completely. We are using DODS and it is clear that we > have different results depending on which type of ICV we use. The > question is... which type of ICV correction for DODS is the most > statistically sound? demean across all groups, no demeaning, or demean > within groups separately. > > What would you recommend. > > On Mon, Aug 29, 2016 at 3:33 PM, Douglas N Greve > <gr...@nmr.mgh.harvard.edu <mailto:gr...@nmr.mgh.harvard.edu>> wrote: > > 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> > <mailto: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> > > <mailto: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> > > <https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer > <https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer>> > > > > -- > > Douglas N. 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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> > > <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 > <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> > > > > > _______________________________________________ > Freesurfer mailing list > Freesurfer@nmr.mgh.harvard.edu > https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer -- Douglas N. 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