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Re: st: Predicted probabilities after Poisson regression

From   Maarten buis <>
Subject   Re: st: Predicted probabilities after Poisson regression
Date   Tue, 6 Jul 2010 18:53:26 +0000 (GMT)

--- On Tue, 6/7/10, wrote:
> Therefore my outcome, low back pain, only ever takes the
> value of zero or  one.  However, the predicted probability
> of Y>1 is greater than zero, even though my outcome is
> only be zero or one. 

-poisson- is a model for counts not binary variables,
so that is  a logical consequence of your model. As long
as the average of your dependent for various values of 
your independent variable stays well away from 1 you can 
use it to get risk ratios, but that only works when the
predicted probability always remains low, otherwise the
assumpion of contant risk ratios is no longer reasonable.

Hope this helps,

Maarten L. Buis
Institut fuer Soziologie
Universitaet Tuebingen
Wilhelmstrasse 36
72074 Tuebingen


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