Hi Chris, There really shouldn't be a "thickness-age correlation group difference" result with the DOSS model. I have FS 4.1 (rather than 5.0 on my system) but running an analogous model, I see that I do indeed get a verbal "Description" for such a contrast. However, if I compare that to the "thickness-age correlation (accounting for group)" result, I see the exact same map. And if you look at the .mat files in the contrast directory generated by qdec, you'll see that those two contrasts are identical (i.e. [0 0 1]) (or at least they are for qdec with FS 4.1).
So, this appears to be a "bug" in the verbal descriptions that qdec provides when using a DOSS model. As to the group difference itself changing between the DOSS and DODS models, that is totally to be expected. Note that in the DODS model, whether or not you demean (center) the age variable has a critical impact on the manner in which you interpret the group contrast. cheers, -MH On Thu, 2011-03-03 at 20:41 -0600, Christopher Bell wrote: > FreeSurfers, > > I have been analyzing my qdec data in version 5.0 and have some > interesting although somewhat confusing results. Basically I have run > a very simple analysis with DODS and DOSS. My discrete factor is group > and my covariate is age, about as simple as can be. > > When I look at the results for DODS I get: > > thickness-age correlation (accounting for group)---result: much of > brain significant > group difference (I assume controlling for age, but it doesn't say > explicitly)--result: one small roi significant > thickness-age correlation group difference--result: one small roi > spatially adjacent to group difference roi > > When I run DOSS I get: > > thickness-age correlation (accounting for group)---result: much of > brain significant > group difference (I assume controlling for age, but it doesn't say > explicitly)--result: much of brain significant > thickness-age correlation group difference--result: much of brain > significant > > I am mostly surprised by how much larger the (group difference), and > the (thickness-age correlation group difference) increase with the > DOSS method. I am also not quite how to interpret the thickness-age > correlation group difference in DOSS. I was thinking the DOSS method > constrained both groups to have the "same slope" and so I was > expecting to get nothing for difference in thickness-age correlation > difference by group; isn't this suggesting my two groups have > significantly different ageXthickness slopes even though they are > constrained to have the same slope by the DOSS method? It would almost > make more sense to me if the results were reversed between DOSS and > DODS. If I had a large thicknessXage correlation group difference > using the DODS method which allows for different slopes. Thanks for > any enlightenment. > > Chris Bell > University of Minnesota > _______________________________________________ > 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. _______________________________________________ Freesurfer mailing list Freesurfer@nmr.mgh.harvard.edu https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer
