Re: regression with interaction of group variable

Posted by Dale Glaser on Sep 08, 2006; 7:32pm
URL: http://spssx-discussion.165.s1.nabble.com/regression-with-interaction-of-group-variable-tp1070835p1070836.html

Marnie..the general (though I'm not sure how universally it is subscribed to) rule is in testing the categorical x continuous variable interaction (see Aiken and West, 1991) is to enter the individual variables (not sure why you call them 'vectors') a 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 is akin to running a fixed-factor 2-way interaction where the interaction is examined over the main effects...........I recall a discussion on this listserv many years ago where someone provided a rationale for only entering the interaction term, but I don't recall what came of the justifcation.

  Dale

Marnie LaNoue <[hidden email]> wrote:
  Hi All -

Any comments/advice appreciated!

I have a situation with 2 groups, and 3 continuous predictor variables. THe
research question is mostly regarding whether the predictors differ across
the groups. I created product vectors of (group x predictor) for all
crossings of group and predictors. Does it make sense to run a regression
with just the product vectors as predictors (ignoring the vectors of
predictor variables?)

Another note: I am using path analysis on this data as well, modeling the
relationships for the groups separately to adress certain predictions
regarding the relationships between predictors and outcomes in the groups,
but I want the regression as an omnibus test of differences in the
predictors. Am I correctly interpreting significant coefficients in the
regression of the product vectors as answering this question?



Dale Glaser, Ph.D.
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