st: Questions about gnpoisson

[kirin_guess@yahoo.com.tw]

Dear all, 
 
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
)
 
Thanks in advance.
 
 
Kind regards, 
 
Chi-lin Tsai

*
*   For searches and help try:
*   http://www.stata.com/help.cgi?search
*   http://www.stata.com/support/statalist/faq
*   http://www.ats.ucla.edu/stat/stata/