Re: [Pymc-user] poisson for negative values

[Christopher F. <ch...@tr...>]

On May 15, 2006, at 7:20 PM, Seth de l'Isle wrote:

> Hello,
>
> I was expecting a poisson prior to limit a parameter to possitive  
> values
> much like using the exponential prior would.  Instead it halted
> processing.
>
>>>> from PyMC.MCMC import *
>>>> class foo(MetropolisHastings): pass
>     ...
>>>> f = foo()
>>>> f.exponential_prior(-1, 5)
>     nan
>>>> f.poisson_prior(-1, 5)
>     PAUSE  negative factorial in factln statement executed
>     To resume execution, type go.  Other input will terminate the job.
>
>     STOP
>
> Shouldn't poisson_prior and poisson_like return nan as well?

Hi Seth,

Thats a good point. At the moment, the likelihoods only enforce the  
parameters of the distribution, and not the data. I essentially leave  
it up to the user not to pass negative data values. Part of the  
reason for this is to avoid as much checking as possible (i.e. "if"  
statements), as this slows down the model.

C.

--
Christopher J. Fonnesbeck, Ph.D.

Population Ecologist, Marine Mammal Section
Fish & Wildlife Research Institute (FWC)
St. Petersburg, FL

Adjunct Assistant Professor
Warnell School of Forestry and Natural Resources
University of Georgia
Athens, GA

T: 727.235.5570
E: chris at trichech.us