Re: regression with interaction of group variable

Posted by statisticsdoc on Sep 08, 2006; 10:49pm
URL: http://spssx-discussion.165.s1.nabble.com/regression-with-interaction-of-group-variable-tp1070835p1070838.html

Stephen Brand
www.statisticsdoc.com

Marnie,

To address the first question, the significance of the cross-product vectors must be assessed when they are entered after the main effects for the continuous and categorical variables.  By cross-product vectors, I mean the interaction terms for the vectors representing levels of the categorical variable times the continuous variables.

The cross-product vectors should be entered as a block - if the block of cross-product vectors is significant, then the significance of the beta for each of the cross-product vectors can be considered (in an equation containing all of the cross-product vectors and main effects).

To address the second question, a significant beta for a single cross-product term (under the conditions described above) indicates that the continuous variable predicts differentially in that group relative to how it predicts for the reference group (depending on how you set up the coding for the categorical variable).

HTH,

Stephen Brand


---- 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?

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