[R] logLik for poisson models

[Charles C. Berry]

On Mon, 2 Feb 2009, Antonio.Gasparrini at lshtm.ac.uk wrote:

> Dear all,
>
> I have a very basic question:
> how does the logLik function work for poisson models?

You mean its inner workings or just what is the computation?

The computation is just the vanilla loglikelihood:

 	sum( dpois( y, mean(y), log=TRUE) )

for your Poisson model.

See
 	?dpois

And for the inner workings, see

 	stats:::logLik.glm

 	poisson()$aic

---

More generally, read the article

   	Ligges, U. (2006): R Help Desk: Accessing the Sources. R News 6
   	(4), 43-45. http://cran.r-project.org/doc/Rnews/

HTH,

Chuck

>
> Example:
> I simulate 20 observations from a Poisson distribution with mean 800.
>
> y <- rpois(20,800)
> model <- glm(y ~ 1, family=poisson())
> logLik(model)
>
> I would like to know what's the exact formula the function logLik uses.
> I looked at ?extractAIC but I cannot sort it out.
>
> Can you please give me an help?
> Thanks a lot
>
> Antonio Gasparrini
> Public and Environmental Health Research Unit (PEHRU)
> London School of Hygiene & Tropical Medicine
> Keppel Street, London WC1E 7HT, UK
> Office: 0044 (0)20 79272406 - Mobile: 0044 (0)79 64925523
> Skype contact: a.gasparrini
> http://www.lshtm.ac.uk/people/gasparrini.antonio ( http://www.lshtm.ac.uk/pehru/ )
>
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Charles C. Berry                            (858) 534-2098
                                             Dept of Family/Preventive Medicine
E mailto:cberry at tajo.ucsd.edu	            UC San Diego
http://famprevmed.ucsd.edu/faculty/cberry/  La Jolla, San Diego 92093-0901