Dear Experts,
I am a little confused by previous posts as how to include confounding
variables in a 3 group TBSS analysis. If you have 3 groups and are
interested in correlations between FA/MD and a behavioural score, and how
these correlations compare between groups, but there are confounding factors
measured in all subjects (which vary within groups and between groups,
though the between group variability is greater) such as age and IQ, how
should they be included ?
Should one:
a) Include each confounder as one ev, demeaned across all 3 groups (would
this account for between group variability in confounders only?)
b) Split each confounder into 3 evs and demean across each Ev before padding
with zeros (would this account for within group variability in confounders
only?)
c) Demean each confounder across all three groups and then split each
confounder into 3 evs (would this account for within group AND between group
variability in confounders ?)
Thanks.
Mahinda
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