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re: st: interactions

From   David Airey <>
Subject   re: st: interactions
Date   Thu, 26 Jun 2003 10:58:45 -0500

Original question
> >> What's the easiest way to generate all [n-way] interactions
> >> when you have many continuous variables to predict a continuous
> >> response?

On Thu, 26 Jun 2003, David Airey wrote:
> I have not been able to use xi to create continuous*continuous
> interactions on the fly. I don't think xi does this from the
> description. It does cat*cont though.

Do cont*cont interactions make sense? It is often an act of faith that a
unit increase in X_i produces a Beta_i increase in Y and that the Betas
are additive. It seems wildly optimistic that a k increase in X_i
together with an l increase in X_j should give the same additional
increase as an l increase in X_i with a k increase in X_j.

This seems in general an approach based on having columns of figures so a
computer will always do the arithmetic, whether or not the sum is

I'm guilty of that last criticism.

Is the reason xi was designed to give the interactions it does based on any truth to your criticism?


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