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Re: st: Poisson model with interaction term

From   Maarten buis <>
Subject   Re: st: Poisson model with interaction term
Date   Wed, 27 Feb 2008 07:26:04 +0000 (GMT)

--- Lloyd Dumont <> wrote:
> Thanks, Maarten.  After a lot of thinking about this
> and looking over which parts of the code generated
> which parts of the graph and trying to recall how
> marginal changes are computed with a logit, I think I
> get your example.  But, I'm still not sure how to
> translate that back to a poisson or negative binomial
> context---and if the panel nature of the data make it
> even more complex.

I see three scenarios: 
1) You are content of interpreting in terms changes in 
   ln(rate) instead of rate. In this scenario there is 
   no need to do anything special.
2) You want to know how the effect of x1 on the rate 
   changes when x2 changes and you don't care whether 
   that change in effect is due to the interaction 
   term or due to change in effect that would occur 
   anyhow in a nonlinear model. You can use the 
   method proposed by Norton and collegues.
3) You want to know how much the interaction effect
   adds on top of changes that would occur anyhow in
   a non-linear model. Now I do not know the answer.

I guess that scenario 2 is by far the most common.

-- Maarten

Maarten L. Buis
Department of Social Research Methodology
Vrije Universiteit Amsterdam
Boelelaan 1081
1081 HV Amsterdam
The Netherlands

visiting address:
Buitenveldertselaan 3 (Metropolitan), room Z434

+31 20 5986715

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