[R] need help with an interaction term

[Chuck Cleland]

Christian Jones wrote:
> Hello!
> I�m fitting a model with glm(family binomial). The best model counts 9 Variables and includes an interaction term that was generated by the product of to continuous variables (a*b). All variables are correlated under a value of 0.7 (Spearman rank order) While the estimates of both main effects are negativ, the resulting interaction term is positiv. This change of sign makes it difficult to interpret the model and above all, is this perhaps due to a bad variable choice ?
> Thanks a lot for helping

  Rather than trying to interpret the model coefficients directly, you
might visualize the a*b interaction effect.  The effects package by John
Fox is very useful for this:

http://cran.r-project.org/doc/packages/effects.pdf

> Christian
> 
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-- 
Chuck Cleland, Ph.D.
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