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Re: st: Overdispersed poisson regression


From   Nick Cox <njcoxstata@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Overdispersed poisson regression
Date   Mon, 5 Sep 2011 12:07:47 +0100

You posted this earlier. As your variance is much less than the mean,
why do you call the data overdispersed?

There are several possible reasons why your earlier mail did not get a
reply, ranging from many people being on vacation to the possibility
that this is not enough information to provide well-grounded advice on
modelling. Perhaps many of your zeros belong in a different group.

Nick

On Mon, Sep 5, 2011 at 11:55 AM, Lynsey Patterson
<Lynsey.Patterson@hscni.net> wrote:

> I am interested in analysing the trend in rates of meticillin resistant S. aureus (MRSA) for my region. I have several variables to include in the model – time (the year (1) and the quarter (2)), the number of episodes (3), the number of occupied bed days (the denominator (4)) and the Trust (a measure of geography (5)).
>
> When I summarize the rate I get a mean of 0.127/1000 bed days with a variance of 0.008. So I think a normal poisson model cannot be fitted and I need to use an over dispersed? I have read about a command for negative binomial regression – does anyone know if this is correct and what syntax I should use?
>
> E.G. nbreg number of episodes (3) year(1) quarter(2) trust(5), exp(occupied bed days (4))
>

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