Message-ID: <87caca24a5a940bab42bf4a8eefad579@BLUPR01MB035.prod.exchangelabs.com>
Date: 2014-02-23T02:00:23Z
From: Dario Strbenac
Subject: Random Count Generation with rnbinom
The documentation states :
An alternative parametrization (often used in ecology) is by the mean ?mu?, and ?size?, the dispersion parameter.
However, this fails :
> rnbinom(10, mu = 100, size = 0)
[1] NaN NaN NaN NaN NaN NaN NaN NaN NaN NaN
Warning message:
In rnbinom(10, mu = 100, size = 0) : NAs produced
For dispersion set to 0, it should work like drawing from a Poisson distribution.
It also produces incorrect draws for non-zero dispersions :
> rnbinom(10, mu = 100, size = 0.000001)
[1] 0 0 0 0 0 0 0 0 0 0
It also fails for underdispersed count models :
> rnbinom(10, mu = 100, size = -0.1)
[1] NaN NaN NaN NaN NaN NaN NaN NaN NaN NaN
Warning message:
In rnbinom(10, mu = 100, size = -0.1) : NAs produced
> sessionInfo()
R version 3.0.2 (2013-09-25)
Platform: x86_64-pc-linux-gnu (64-bit)
locale:
[1] LC_CTYPE=en_AU.UTF-8 LC_NUMERIC=C
[3] LC_TIME=en_AU.UTF-8 LC_COLLATE=en_AU.UTF-8
[5] LC_MONETARY=en_AU.UTF-8 LC_MESSAGES=en_AU.UTF-8
[7] LC_PAPER=en_AU.UTF-8 LC_NAME=C
[9] LC_ADDRESS=C LC_TELEPHONE=C
[11] LC_MEASUREMENT=en_AU.UTF-8 LC_IDENTIFICATION=C
attached base packages:
[1] stats graphics grDevices utils datasets methods base
loaded via a namespace (and not attached):
[1] tools_3.0.2
--------------------------------------
Dario Strbenac
PhD Student
University of Sydney
Camperdown NSW 2050
Australia