Re: Interaction with continuous variables

Posted by Ryan on May 02, 2013; 2:12am
URL: http://spssx-discussion.165.s1.nabble.com/Interaction-with-continuous-variables-tp5719846p5719874.html

I was also concerned when I read that statement, along with "hierarchical" regression. As Bruce said, make sure all lower-order terms are in the final model from which you will obtain the fitted values.

 

@Bruce: I was wondering what was taking you so long to respond to this post. This is right up your alley, my friend, especially given how quiet it is these days on SPSS-L...

 

:-)

 

Ryan

On Wed, May 1, 2013 at 9:54 PM, Bruce Weaver <[hidden email]> wrote:

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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