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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