Re: regression with interaction of group variable

Posted by Richard Ristow on Sep 09, 2006; 12:09am
URL: http://spssx-discussion.165.s1.nabble.com/regression-with-interaction-of-group-variable-tp1070835p1070837.html

A general comment; Dale Glaser's post is just a useful starting point.

At 02:32 PM 9/8/2006, Dale Glaser wrote:

>The general rule is in testing the categorical x continuous variable
>interaction (see Aiken and West, 1991) is to enter the individual
>variables at the first step and then the multiplicative term at the
>subsequent step of entry, and then examine the incremental statistics
>to assess if the interaction term added variance above and beyond the
>constituent variables (also, this assumes you have centered the
>continuous level predictor(s)).

This may go without saying, but it doesn't seem to have been said
explicitly: When you enter the category level x continuous variables,
also enter the category indicator variable, or variables; but do NOT
include the category indicator variables in the group test. That avoids
a test that's partly testing whether the two groups have different mean
values of the dependent.

That may be what "centering the continuous predictors" is meant to
avoid, as well. Unless I'm missing something badly, I'd think it
easier, and more reliable, to enter the level indicator variables (it
amounts to letting the 'constant' be adjusted between levels), and
don't worry about centering the continuous predictors.