two queries on distribution

Ottar N. Bjornstad (
Sat, 03 Jan 1998 11:46:20 +0100

Dear All,

I have two questions about the gamma distribution and the negative binomial distribution (in Splus 4):

1) Why does the Gamma distribution only have 1 parameter (the shape parameter) in Splus, when the usual Gamma is a two parameter distribution?

2) I'm really confused about the Negative binomial:
- Venables & Ripley provided a function rnegbin that takes two parameters: the mean and the shape parameter.
- Splus 4 has a function for the negative binomial called rnbinom that takes two parameters (size, and probability). What is the relationship between the two? can I use the new rnbinom like I use the rnegbin - to generate overdisperesed poisson data (by specifying the shape parameter)?

Gratefully yours,
Ottar Bjornstad

Ottar N. Bjornstad
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