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st: Re: Reporting Gamma Regression


From   Joseph Coveney <jcoveney@bigplanet.com>
To   Statalist <statalist@hsphsun2.harvard.edu>
Subject   st: Re: Reporting Gamma Regression
Date   Tue, 10 Aug 2004 00:44:43 +0900

Stephen Soldz wrote:

> A non-Stata question, though I'm using Stata for the anlysis. I'm analysing
> cost data and have used a GLM with gamma family and log link. The question I
> have is regards reporyting results. Should I exponentiate the coefficients
> (eform)? If so, any suggestions on how to describe this? Does anyone know of
> published examples I could refer to?

Did you ever get an answer?  

The examples that crop up on the Web tend to either leave the coefficients 
unexponentiated ( http://harrisschool.uchicago.edu/pdf/wp_03_13.pdf this is 
Willard Manning, Anirban Basu & John Mullahy) or seem to use predictions (mu) 
and talk about changes in them 
( www.journals.uchicago.edu/CID/journal/issues/v33n12/001639/001639.text.html 
and www.dianthus.co.uk/resources/sampleReport.pdf , and maybe 
www.academyhealth.org/arm/abstracts/medicare.pdf ).  

Log-gamma models are discussed in J. Hardin & J. Hilbe, _Generalized Linear 
Models and Extensions_ (College Station, Texas: Stata Corporation, 2001), 
pp. 69-71.  They, too, didn't exponentiate the coefficients.  

The one example of exponentiated coefficients that turned up (not exhaustive) 
( www.mrc-bsu.cam.ac.uk/BSUsite/Publications/Preprints/glm_p6.pdf ) didn't give 
them a very snazzy name, analogous to risk ratios or incidence rate ratios.

Joseph Coveney


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