Dear all,
I have two questions concerning 2nd level multiple regression/correlation
analysis, and hope to get some advice:
1) What is the difference between multiple regression and multiple
regression with constant? Which one should I use when I want to asses
the effects of two predictors (scores of questionnaires) on contrast images
(results of fixed effects analysis)
2) how could I explain the following result:
I have two independent predictors (scores of two questionnaire data),
and want to know whether they correlate with activation data (contrasts
of a fixed effects analysis, N=17 subjects).
When I analyse the scores independently using two simpleregression
(correlation) analyses, I get almost no effect for score1;
however, when I do a multiple regression analysis (no constant),
I get a huge effect for both scores together (assessed using an F contrast),
which is mainly attributable to score1
is this because I don't have a constant term, so the significant voxels
are simply reflecting the mean activity in the whole sample?
thanks for your help
claus
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