Re: st: Overdispersed poisson regression

[Muhammad Anees <anees@aneconomist.com>]

Lynsey!

I would suggest you read the earlier posts on the similar topics by
googling your problems plus stata like (count data stata) or (poisson
models stata) and surely without the paranthesis, which will search
out a complete discussions using stata for you. For more details a
good website for learning stata is, in general

http://www.ats.ucla.edu/stat/stata/topics

and specially on count data models, its other link

http://www.ats.ucla.edu/stat/stata/topics/CountModels.htm.

I hope this works all out for you.

Anees

On Mon, Sep 5, 2011 at 4:16 PM, Lynsey Patterson
<Lynsey.Patterson@hscni.net> wrote:
> âThis email is covered by the disclaimer found at the end of the message.â
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> ________________________________
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>
> Sorry Nick,
>
> I am new to this list and I got several bounces saying the message had not been posted. I tried to find it to check and thought to be safe I would try again.
>
> I was using the term over dispersed as I know this has been applied to data from other administrations.
>
> Basically, I am trying to look at the trend in MRSA rates - hence the choice of Poisson. I then need to decide which model best fits the data - sorry for the confusion, evidently I'm not a statistician!!
>
> Best wishes,
>
> Lynsey
>
> -----Original Message-----
> From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Nick Cox
> Sent: 05 September 2011 12:08
> To: statalist@hsphsun2.harvard.edu
> Subject: Re: st: Overdispersed poisson regression
>
> 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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-- 


Regards

Anees

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