I'm conducting a tbss analysis of radial diffusivity between two groups (normal versus depressed) and want to include anhedonia (continuous scale variable) as a covariate. I am wondering if I should demean the anhedonia EV separately between groups or not, given that the mean anhedonia score is markedly different between the two groups, normal versus depressed.
I've read through some of the other threads about demeaning across vs between groups and still am not quite clear on this topic.
Thanks in advance for any help!
-Genevieve
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