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Re: Interaction with continuous variables

Posted by Bruce Weaver on May 02, 2013; 1:54am
URL: http://spssx-discussion.165.s1.nabble.com/Interaction-with-continuous-variables-tp5719846p5719873.html

I just noticed this post now.  In a nutshell, what you want to do is save fitted values of Y at several selected combinations of the two interacting variables, and then plot them.  (This will yield a graph that is quite similar to what you'd get for a two-way ANOVA.)

For more information, I recommend that you check your library to see if it has the 1991 book by Aiken & West on interactions in regression models.  I also have some notes I could send.  If you want them, send me a message at the e-mail address shown in my signature file below.  (The hotmail address I use for posting to this list is not my everyday e-mail, and I do not check it very often.)

Aiken, L. S., & West, S. G. (1991). Multiple Regression: Testing and interpreting interactions. Thousand Oaks: Sage.

Oh, one other thing:  You said "the main effects are included in an earlier step of the model".  It's not clear if that means you took them out after adding the product term.  If you did take them out, you should put them back in.  For any two-way interaction, you should include both first-order terms; for any 3-way interaction, you should include all 3 first order terms, and all possible 2-way interactions involving those first-order terms.  And so on.

HTH.


NomiW wrote
Hi All,
I have run a hierarchical linear regression model which includes an
interaction term between 2 continuous variables. The variables are centered
and the main effects are included in an earlier step of the model.
The interaction is significant but I'm not sure how to interpret this. Any
suggestion of a way to graph (or break down) this interaction would be
appreciated.
Thanks,


Nomi Werbeloff, PhD

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Bruce Weaver
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