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Re: st: Questions about gnpoisson

From   Nick Cox <>
Subject   Re: st: Questions about gnpoisson
Date   Thu, 20 Sep 2012 18:55:17 +0100

-gnpoisson- is a user-written command from SSC, as you explain.

I imagine that by phi is meant ln(phi) in the passage you cite.

The help file for -gnpoisson- gives references to two books, which no
doubt say more.

Please don't give partial name (date) references; this is an explicit
request in the FAQ.


On Thu, Sep 20, 2012 at 6:13 PM,  <> wrote:

> I'm using "gnpoisson", and having two questions about it.
> Firstly, according the help file of gnpoisson:
> “Positive values of phi adjust for Poisson overdispersion;
> negative values adjust for Poisson under-dispersion.”
> However, gnpoisson estimates ln(phi),
> and calculates the phi for the model output via exp(ln(phi)).
> Given this re-parameterisation, how can the phi be negative?
> Secondly, there are at least two kinds of generalised poisson
> model (Yang, Hardin, and Addy 2009), what kind of generalised
> poisson model is gnpoisson based on?
> (By the way, to install gnpoisson, just type:
> ssc install gnpoisson

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