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Hello,
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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