If x1 and x2 are the same thing, then the equation is non-invertible. If they are the not the same variable, then there is no problem. QDEC computes all the contrasts for you (but excludes contrasts with nuisance variables) doug
On 8/13/13 3:03 PM, ye tian wrote:
Dear Doug,
I am afraid that I still don't understand why a variable can not be "covarying" and "nuisance" simultaneously.
If variables x1 and x2 are covarying, they both go into the model y = b0 + x1*b1 + x2*b2 + err Is this correct?
Nuisance, however, is related to the contrast, i.e., test hypothesis. If x2 is nuisance, then (1 0) * (x1 x2) = 0 Is this correct?
Otherwise, how does one specify the contrast matrix in Qdec?
Thank you very much!
Sincerely, Ye
On Tue, Aug 13, 2013 at 12:03 PM, Douglas N Greve <greve@nmr.mgh.harvard.edu mailto:greve@nmr.mgh.harvard.edu> wrote:
Hi Ye, don't select the same factor as nuisance and covariate. It will result in an error for sure. The difference between a covariate and a "nuisance" factor is purely conceptual. By calling something a nuisance you tell QDEC not to create a contrast related to it. doug On 08/13/2013 02:09 AM, ye tian wrote: > Dear Freesurfers, > I have a question concerning the "nuisance factors" in the qdec > dialogue. Suppose I have two independent variables x1 (discrete) and > x2 (continuous), and only one observable y. I choose "Discrete (Fixed > factor) = x1", "Continuous (covariate) = x2". > If I further chose "Nuisance factor = x2", does that imply that > 1) my contrast matrix is (0 1) > 2) null hypothesis: Y is independent of x2 > Thank you very much! > Sincerely, > Ye > > > _______________________________________________ > Freesurfer mailing list > Freesurfer@nmr.mgh.harvard.edu <mailto:Freesurfer@nmr.mgh.harvard.edu> > 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 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/ _______________________________________________ Freesurfer mailing list Freesurfer@nmr.mgh.harvard.edu <mailto: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.