Dear Doug, Thanks again for your reply. Based on that I did some further work.
I first demeaned the age of all subjects. Actually, I have a third group which I would like to compare to, so my contrast matrices will be [.5 .5 -.5 -.5 0 0 0 0 0 0 0 0] [0 0 .5 .5 -.5 -.5 0 0 0 0 0 0] [.5 .5 0 0 -.5 -.5 0 0 0 0 0 0] to test for CT differences between all the groups while correcting for age and sex. Surprisingly, I'm observing a big difference in the results compared to the results without demeaning. Could you explain the reson for this? In the FSGD-examples (eg http://surfer.nmr.mgh.harvard.edu/fswiki/FsgdFormat), age is also not normalized. Does normalizing the variance to 1 also influence the results? Given this big difference, I started wondering whether it would maybe be better to analyze the data in pairs of two groups (and then demean by the mean of the two groups). Would this be a better approach? Concerning your second suggestion: if I test the data for differences in group slope, a number of small area's are significantly different. Regions popping up are especially in the neighborhood of the insula. Unfortunately this suggests that I cannot use the DOSS model, or am I wrong? Looking forward to your reply, With best regards, Martijn On Sat, Dec 10, 2011 at 7:16 PM, Douglas Greve <gr...@nmr.mgh.harvard.edu>wrote: > Yes, that is correct, though I think your matrix should be [.5 .5 -.5 -.5 > 0 0 0 0]. You should also remove the mean from the age (mean computed from > all subjects). Or even better, first test whether there is a group > difference in age slope with [0 0 0 0 .5 .5 -.5 -.5]. If there is nothing > that is significant, then re-run your analysis using the Different Offset > Same Slope (DOSS) model with this contrast [.5 .5 -.5 -.5 0]. > > doug > > > On 12/10/11 4:15 AM, Martijn Steenwijk wrote: > > Dear all,**** > > ** ** > > I’m relatively new with Freesurfer, but slowly getting more and more used > to it’s great possibilities. To be ‘sure’, I’ve a question about the design > of a GLM. **** > > ** ** > > I want to compare CT in Healthy Controls vs Diseased, and control for age > and sex. It appears to me that factors (eg sex) cannot be used as > covariate/variable, which forces me to model them as a separate class > although I’m not interested in sex differences. This brings me to the > following FSGD file:**** > > ** ** > > # HcDis.fsgd**** > > GroupDescriptorFile 1**** > > Title HcDis**** > > Class Hc_Male**** > > Class Hc_Female**** > > Class Dis_Male**** > > Class Dis_Female**** > > Variables Age**** > > Input subjid1 Hc_Male 35**** > > Input subjid2 Dis_Female 30**** > > ….**** > > ** ** > > Then the difference between Hc and Dis, corrected for age and sex is given > by the contrast matrix**** > > #Hc-vs-Dis.mtx**** > > 0.5 0.5 -0.5 -0.5 0 0 0 0 0 0 0 0 **** > > ** ** > > Is this correct? **** > > ** ** > > Best,**** > > Martijn**** > > ** ** > > ** ** > > > _______________________________________________ > Freesurfer mailing > listfreesur...@nmr.mgh.harvard.eduhttps://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 > > > 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. > >
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