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Re: st: Adjusting for clustering within groups


From   Maarten buis <maartenbuis@yahoo.co.uk>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Adjusting for clustering within groups
Date   Tue, 8 Apr 2008 12:45:39 +0100 (BST)

--- Ashwin Ananthakrishnan <ashwinna@yahoo.com> wrote:
> I am currently using Stata 9.2. My analysis involves
> logistic regression of a dichotomous outcome (dead /
> alive) across various predictors. My main predictor of
> interest is hospital type (Small, medium, big). How do
> I adjust for clustering within each type of hospital? 

If I would do a control for clustering than I would 
control for clustering within hospital, not within type
of hospital. 

> The largest of the 'big' hospitals may contribute more
> patients to the analysis than the smaller of the 'big'
> hospitals. Do I need to adjust for this phenomenon?
> i.e. if we hypothesize that larger hospitals have
> better outcomes, isn't it appropriate that the larger
> of the big hospitals contribute more patients and
> hence have a larger weight?
> 
> Or do I need to adjust for this? and how do I do it?

Differences in size of groups influence the standard 
errors. Generally speaking: an odds ratio comparing 
two large groups will have a smaller standard error 
than an odds ratio comparing a large and a small 
group, which turn will have a smaller standard error
than the odds ratio comparing two small groups. This 
is exactly as it should be (more data --> less 
uncertainty --> smaller standard error), so there is 
no need for any adjustement here.

Hope this helps,
Maarten

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

visiting address:
Buitenveldertselaan 3 (Metropolitan), room Z434

+31 20 5986715

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


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