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Re: st: Predicted probabilities after Poisson regression


From   Gillian.Frost@hsl.gov.uk
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Predicted probabilities after Poisson regression
Date   Wed, 7 Jul 2010 08:57:09 +0100

Thank you very much for your responses.  You have been extremely helpful.

Many thanks,

Gillian





Maarten buis <maartenbuis@yahoo.co.uk> 
Sent by: owner-statalist@hsphsun2.harvard.edu
06/07/2010 19:53
Please respond to
statalist@hsphsun2.harvard.edu


To
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cc

Subject
Re: st: Predicted probabilities after Poisson regression






--- On Tue, 6/7/10, Gillian.Frost@hsl.gov.uk 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 

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

http://www.maartenbuis.nl
--------------------------


 

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