You should model sex as a categorical factor, not as a continuous variable, so there is no issue of centering sex. Whether you really need to center age depends on what you want to test. If you want to test of a difference in age slopes regressing out the effects of sex, then you don't need to change the age values. If you want to test for a difference in sex regressing out the effects of age, then you should first test for a difference in age slopes as described above. If there are no differences, then you should re-run the analysis using DOSS (different offset, same slope) in which case you do not need to demean the age. doug
On 10/05/2012 04:58 PM, Yang, Daniel wrote: > Dear freesurfer experts, > > I'm using Qdec and I want to control for age (a continuous variable) > and sex (f,m or 1,0). > > Do I need to center age (i.e., subtract the score from its mean) > before I select it as a nuisance factor? > Do I need to center sex? > > I found that selecting age_centered as a nuisance factor vs. selecting > age_uncentered as a nuisance factor led to very different results… not > sure what is the correct way to go. > In brief, if I want to control for age and sex, which of the following > is the correct way to go? > > 1. Center neither age nor sex (f,m). Then choose age_uncentered as a > nuisance factor, but sex_uncentered as a discrete/fixed factor. > 2. Center age but not sex (f,m). Then choose age_centered as a > nuisance factor, but sex_uncentered as a discrete/fixed factor. > 3. Center age but not sex (1,0). Then choose both age_centered and > sex_uncentered as nuisance factors. > 4. Center age and sex (1,0). Then choose both age_centered and > sex_centered as nuisance factors. > > Thanks in advance! > Daniel > -- > Yung-Jui "Daniel" Yang, PhD > Postdoctoral Researcher > Yale Child Study Center > New Haven, CT > (203) 737-5454 > > > _______________________________________________ > 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: www.nmr.mgh.harvard.edu/facility/filedrop/index.html _______________________________________________ Freesurfer mailing list Freesurfer@nmr.mgh.harvard.edu 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 . 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.
