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I am looking to run a MANCOVA to examine cortical gray matter differences across three groups (accounting for several covariates), and I was hoping to get input on whether I am setting up my design matrix correctly. Groups: 3 Covariates: Age (demeaned), disease duration (DD; demeaned), sex Simplified Example: Group EV1(Grp1) EV2(Grp2) EV3(Grp3) EV4(age) EV5(DD) EV6(sex) 1 1 0 0 0.25 0.75 -1 1 1 0 0 0.25 0.25 -1 1 0 1 0 -0.75 -0.25 1 1 0 1 0 0.5 -0.75 1 1 0 0 1 -0.5 0.5 1 1 0 0 1 0.25 0.25 -1 Questions: 1. Is this an appropriate way to do a three-group comparison with covariates? Or, is it better to just run three separate t-tests (including covariates)? 2. Regarding post-hoc testing, would I just do three separate t-tests? This issue here, I imagine, is that the individual t-test results may not reflect the original MANCOVA results (e.g., post-hoc testing reveals a new significant area that was not significant in the MANCOVA). 3. One other option is to run the initial MANCOVA within a statistical program (i.e., using global cortical gray matter volumes) and then follow up with postdoc testing within FSL (i.e., t-tests). Would this make sense? Thanks in advance for your time and help, Cristina
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