Hello,
I have several questions about setting up a HLA with covariates (of
interest). I have a single group, with scores from 3 different
questionnaires (A, B, & C). I would like to know which brain areas are
associated with responses on each questionnaire (for a given lower-level
contrast). Questions below:
1) Am I correct that each questionnaire score needs to be demeaned
(separately for each questionnaire)? If I am not interested in the group
mean (EV1), is it necessary to demean the scores for each questionnaire,
or can I input the raw questionnaire scores?
Assuming I would need to demean the scores, this is how I would set up the
EVs and contrasts (using a small number of subjects as an example).
EVs:
Group EV1(group mean) EV2(Ques A) EV3(Ques B) EV4(Ques C)
1 1 1.4 3.2 3.2
1 1 5.4 .2 1.2
1 1 2.4 -1.8 -3.8
1 1 -4.6 3.2 -3.8
1 1 -4.6 -4.8 3.2
Contrasts:
Contrast EV1 EV2 EV3 EV4
Questionnaire A 0 1 0 0
Questionnaire B 0 0 1 0
Questionnaire C 0 0 0 1
2) If in addition I am interested in looking at the interaction effects, I
think I would add 4 more EVs to this model (consisting of the following
multiplied scores) set up the orthogonalizations as specified below, and
add 4 corresponding contrasts. Does this look like the correct way to set
up the orthogonalizations (and interactions)?
Add 4 more EVs:
EV5 = QuestionnaireA * QuestionnaireB
EV6 = QuestionnaireA * QuestionnaireC
EV7 = QuestionnaireB * QuestionnaireC
EV8 = QuestionnaireA* QuestionnaireB * QuestionnaireC
Set up orthogonalizations:
Orthogonalize EV5 wrt EV2, EV3, & EV4
Orthogonalize EV6 wrt EV2, EV3, & EV4
Orthogonalize EV7 wrt EV2, EV3, & EV4
Orthogonalize EV8 wrt EV2, EV3, EV4, EV5, EV6, & EV7
Add 4 more contrasts:
Contrast EV5 EV6 EV7 EV8
QuestionnaireA*B 1 0 0 0
QuestionnaireA*C 0 1 0 0
QuesionnaireB*C 0 0 1 0
QuestionnaireA*B*C 0 0 0 1
3) If the questionnaires are not on the same scale, is it necessary to
transform the scores to z-scores? If we do not z-score the questionnaire
scores, are the interactions interpretable?
Thanks for your help
~Anna
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