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Thanks, Yingying. The two groups are controls and patients and we are interested in diffusion metrics associated with the condition and not due to developmental changes (age). What does demeaning age within groups (for controls and patients separately) tell us and what would demeaning all subjects (patients and controls) with the grand mean tell us? Many thanks, --Heather On Tue, Dec 20, 2011 at 12:00 PM, Yingying Wang <[log in to unmask]>wrote: > Hi Heather, > > According to your message, you might want to do the two group separately. > Also it depends on your hypothesis. > > For example: two group: controls and patients > What's your interest? > > 1) Use age as covarite (nuisance variable) if you're not interested in > developmental changes. > > 2) http://mumford.fmripower.org/mean_centering/ > > Best, Yingying > > >>> Heather Collins <[log in to unmask]> 12/20/2011 9:50 AM >>> > Dear FSL experts, > > I am running TBSS on 2 groups of subjects. The mean ages are not > different between the groups. However, when any diffusion metric is > plotted over age, the groups show different slopes: one increases with age > whereas the other decreases with age. (It looks like an X) > > I have combed through the archives and cannot find information about this > kind of situation. > > 1) Should age be used as a covarite in this situation? > > 2) What does it mean for age to be demeaned within each group versus > demeaning both groups using the grand mean? Which should I use in this > situation? > > Thank you in advance for your time and help! > > Cheers, > > --Heather >