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Re: st: Adjusting for clustering within groups

From   Maarten buis <>
Subject   Re: st: Adjusting for clustering within groups
Date   Tue, 8 Apr 2008 12:45:39 +0100 (BST)

--- Ashwin Ananthakrishnan <> 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 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

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