[Rd] dnbinom with a large size parameter (PR#13650)

[Peter Dalgaard]

4ap1 at queensu.ca wrote:
> Full_Name: Andrey Pavlov
> Version: 2.7.1 (2008-06-23)
!!!


> OS: Windows Vista
> Submission from: (NULL) (67.193.233.43)
> 
> 
> Dear developers,
> 
> I discovered an issue with the dnbinom function while fitting a negative
> binomial model to my data. I was using the size and mu parameterization. When
> the size gets large enough, the function begins to return 1, while it should
> instead return the respective Poisson probability. This can be seen in the
> following simple example:
> 
>> dpois(1,lambda=1)
> [1] 0.3678794
>> dnbinom(1,size=1e+15,mu=1)
> [1] 0.3678793
>> dnbinom(1,size=3e+15,mu=1)
> [1] 0.3678793
> 
> - very close to Poisson. But then I increase the size further, and it goes off
> somewhat:
> 
>> dnbinom(1,size=5e+15,mu=1)
> [1] 0.3658024
>> dnbinom(1,size=7e+15,mu=1)
> [1] 0.3572676
> 
> ...until suddenly it returns 1:
>> dnbinom(1,size=10e+15,mu=1)
> [1] 1
> 
> This turned out to be a big and hard to track down issue for me. The bug
> confused the optimizer of the likelihood function, which happened to move too
> far on the size dimension and began to discover "very good" parameters. I fixed
> the problem by adding a logical check, which replaced the negative binomial
> probability with the Poisson one in case the size was large, but this is a very
> crude solution. Perhaps it is worth fixing the internal routine.

Doesn't happen in 2.8.1:

 > dnbinom(1,size=10e+15,mu=1)
[1] 0.3678794

This was fixed (if you define it as a "bug" rather than an "FP accuracy 
issue") in 2.7.2/2.7.2patched.

You might at least have checked the NEWS file for references to 
dnbinom() before submitting a bug report on an older version!


-- 
    O__  ---- Peter Dalgaard             Øster Farimagsgade 5, Entr.B
   c/ /'_ --- Dept. of Biostatistics     PO Box 2099, 1014 Cph. K
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