[R] strange behaviour of negative binomial

[Peter Dalgaard]

dax42 <Dax42 at web.de> writes:

> Dear list,
> 
> I ran into a strange behaviour of the pnbinom function - or maybe I
> just made a stupid mistake.
> First thing is that pnbinom seems to be very slow. The other - more
> interesting one - is that I get two different curves when I plot the
> estimated density and the density given by pnbinom. Shouldn't it be
> the same?
> This is only the case, I think, if I use the parameter size = 1. I
> just tried it for size = 2 and the result seemed correct..
> 
> Anyways, this is what I tried:
> 
> #parameters
> proba<-0.001039302;
> s<-1;
> 
> #random numbers with this distribution
> dom<-rnbinom(10000,s,proba);
> 
> #first plot
> plot(ecdf(dom),do.points=FALSE);
> 
> #second plot
> x<-seq(0,8700,0.5);
> lines(x,pnbinom(x,s,proba),col="red");

Errrr... What's the problem here? The two curves look quite similar to
me. (And both are of course cumulative distribution functions and not
densities, but it doesn't sound like that is the problem.) 

-- 
   O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
 (*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)             FAX: (+45) 35327907