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st: RE: Logistic regression: perfect prediction of outcome in Stata9.2


From   "Maarten Buis" <M.Buis@fsw.vu.nl>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: RE: Logistic regression: perfect prediction of outcome in Stata9.2
Date   Mon, 22 May 2006 11:57:11 +0200

--- Heather Rothwell wrote:
> It's important to compare pupils who say all their friends 
> are smokers (3) and those who say some/a few friends are
> smokers (2) , with those who say none of their friends are
> smokers (1) * not just to compare the risks of pupils in
> categories 2 and 3 - so that the variable can be used later
> to adjust for these data in a multivariable model, using
> all the observations.
>
> Is it acceptable to choose one observation from category 1
> at random, and code it 1 for outcome, in order to obtain
> odds ratio estimates for categories 2 and 3?

Heather:
I like simple solutions, so I would do that before doing 
anything more fancy. If you want to try more fancy stuff (or
if a reviewer tells you to do so) you can look at stuff
written by Gary King some time ago on the analysis of rare
events (which fortunately appeared to be the case for 
smoking). You can find some papers and Stata software on
this subject at 
http://gking.harvard.edu/projects/rareevents.shtml . However 
Gary King has converted to R, so he probably won't maintain 
the Stata programs any longer. 

Comparing logistic regressions when adding covariates is 
tricky, see for instance Richard William's notes on
http://www.nd.edu/~rwilliam/xsoc694/x04.pdf

HTH,
Maarten

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

visiting adress:
Buitenveldertselaan 3 (Metropolitan), room Z214 

+31 20 5986715

http://home.fsw.vu.nl/m.buis/
-----------------------------------------


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