Hi,
I have a problem similar to the one entitled, "Re: Help with interaction
matrix in FEAT" from May 2, 2009. I want to model interaction effects in
randomise for a VBM analysis. I have 2 groups (alc, control), 2 sexes (boys,
girls), and 2 nuisance variables. I want to test (1) the main effects of
group membership and (2) sex, along with (3) the group*sex interaction. The
interaction effect will show me if one sex is more susceptible to the
effects of heavy drinking than the other. From what I've read, I need to use
two real EVs for each of my 2 original EVs (group and sex) because this
isn't a repeated measures or paired design. The numbers of
alcoholics/controls and boys/girls are not exactly balanced.
This is how I set up my design matrix, assuming 1 subject per group.
Design:
alc/boy alc/girl con/boy con/girl nuisance_demeaned
1 0 0 0 x
0 1 0 0 x
0 0 1 0 x
0 0 0 1 x
I set up my contrast to include [(con/boy) – (alc/boy)] and [(con/girl) –
(alc/girl)] in an F-test.
Contrast:
alc/boy alc/girl con/boy con/girl nuisance F1
C1 1 1 -1 -1 0 alc>con?
C2 -1 -1 1 1 0 con>alc?
C3 0 -1 0 1 0 x
con/girl>alc/girl?
C4 -1 0 1 0 0 x
con/boy>alc/boy?
Will F1 will test if the volume loss due to alcohol dependence is different
between boys and girls?
If anyone can offer me some help modeling group, sex, and a group*sex
interaction in a design matrix, it would be a big help!
Thanks,
Ryan
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