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Re: st: RE: Transforming regressors prior to estimation in count models


From   Rkaminski@aol.com
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: RE: Transforming regressors prior to estimation in count models
Date   Tue, 7 Jun 2005 16:33:44 EDT

Carter and Richard,
 
Let me clarify that they were estimating count models (not OLS), and that a  
rationale for why they started with transformed variables was not given  
(theoretical or otherwise).
 
Bob
 
In a message dated 6/7/2005 4:24:57 P.M. Eastern Standard Time,  
CRees1@uwyo.edu writes:
Bob,

Was their model also a count model or did  they transform to meet the
normality assumption of  OLS?

Carter
------------------------------------------------------------------------------
----------------------
 
In a message dated 6/7/2005 4:19:56 P.M. Eastern Standard Time, 
_Richard.A.Williams.5@ND.edu_ (mailto:Richard.A.Williams.5@ND.edu)   writes:

First off, I'm not aware of anything "special" about count models when it  
comes to transformations of the regressors, e.g. I don't know why your  
strategy would inherently be different for count models than, say, for  
OLS.  But, there are lots of things I am not aware of, so maybe  somebody 
can offer more insights on this.
 
In general, it seems to me that variable transformations are driven by  
theoretical and/or empirical concerns.  Theory might argue that you  should 
take logs, squares or whatever; and if theory is ambivalent or silent  then 
observed empirical relationships may guide the choice.
 
If you are in the theory is silent category - then I would probably proceed  
as you say.  There are all sorts of ways to transform variables, so I  don't 
think I would try them all out just to see what happens.  But, if  you are 
replicating someone else's work, they may have had good reasons for  doing 
what they did, so theory may not be silent here.
 

-----Original Message-----
From: Rkaminski@aol.com  [mailto:Rkaminski@aol.com] 
Sent: Tuesday, June 07, 2005 1:44 PM
To:  statalist@hsphsun2.harvard.edu
Subject: st: Transforming regressors prior to  estimation in count models

Dear All,

I'm about to embark on  an  analysis using count models to replicate a
study 
in which the  authors log  transformed or used some other transformation
on 
their  regressors prior to  estimating their models. I was under the
impression  
that typically one begins  with untransformed variables in count  models,
assess 
the fit, and then -- if  necessary -- transforms  regressors. 

Any thoughts on these  approaches?

Thanks in  advance.

Bob Kaminski

Department of  Criminology and Criminal  Justice
Currell College
University of South   Carolina
kaminskb@gwm.sc.edu
(803) 777-1560   

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