From ripley@stats.ox.ac.uk  Fri Apr  9 08:48:25 1999
From: ripley@stats.ox.ac.uk (ripley@stats.ox.ac.uk)
Date: Fri, 9 Apr 1999 09:48:25 +0200
Subject: Error in ppois function (PR#161)
Message-ID: <199904090748.JAA00884@blueberry.kubism.ku.dk>

On Fri, 9 Apr 1999 ihaka@stat.auckland.ac.nz wrote:

> On Fri, 9 Apr 1999, Prof Brian D Ripley wrote:
> 
> >      ppois(q, lambda)
> > 
> > where q is undefined! It should only be defined for integers. Similarly, x
> > should only be integers.
> 
> Ummmm.  I guess I see the argument for dpois, but I thought the
> distribution function was defined for all values.  Wouldn't it be in order
> to apply the integer argument function to floor(q) perhaps?

Try reading on:

> > It seems better to make ppois defined as the CDF for all q. The code
had
> > 
> >    x = floor(x + 0.5);
> >
> > and that should be something like floor(x + 1e-7).

(q in R is x in C.)

> This seems to be what S does.
> 
> 	> ppois(1.5, 2)
> 	[1] 0.4060058
> 	> ppois(1.1, 2)
> 	[1] 0.4060058

and R-0.64.1 does

> ppois(1.5, 2)
[1] 0.4060058
> ppois(1.1, 2)
[1] 0.4060058
> dpois(1.1, 2)
Warning: non-integer x = 1.100000
[1] 0

Happy?

Brian


-- 
Brian D. Ripley,                  ripley@stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272860 (secr)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595


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Error in ppois function (PR#161)
ripley@stats.ox.ac.uk