Dear SPM experts,
I have a question regarding the use of the term "interaction" when SPM5
sets up a factorial model on the second level.
As I have understood interactions between two factors are modelled by
entering say 1 for factor 1 and -1 for factor 2 into the contrast manager.
But how does this relate to the interaction terms used in a multiple
regression model when one uses a statistical software (e.g. SPSS).
I always thought the interaction term would normally be a multiplication
of two predictors, which have to be estimated by a least square solution
in the multiple regression model.
What I do, when I enter 1 -1 for factor 1 and factor 2 respectively into
the contrast manager in SPM, however, is to look for inverse partial or
adjusted effects of the two factors (since as I have understood SPM always
yields partial effects adjusted for everything else in the model),
correct ?
I don't quite understand how both ways of modelling an interaction relates
to one another, or did I get the way how to model interactions in SPM
wrong ?
Can anybody help to dissolve my confusion ?
I must have an error of understanding somewhere ....
Thanks a lot for any hint !!
Best,
Roland
Roland Zahn, Dr. med.
NIH / NINDS
Cognitive Neuroscience Section
Building 10, 5C206
10 Center Drive, MSC 1440
Bethesda, MD 20892-1440
Tel.: (+1)-301-402-6392
Fax.: (+1)-301-480-2909
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