[R] Poisson distribution help requested

Thomas Lumley tlumley at u.washington.edu
Fri Jan 9 17:22:27 CET 2004

On Fri, 9 Jan 2004, Mark St.John wrote:

> Could somebody help me to understand the syntax of R's ppois function?
> I'm looking to calculate the cumulative probability density of an
> observed value (y) given the expected mean (mu) and the level of
> significance (alpha). I'm coming from using SAS to do this and don't
> recognize the descriptions of the arguments for ppois. The definitions
> of lambda and p as stated in the R manuals are foreign to me!

You can't specify y and mu and alpha simultaneously. It's not clear what
you want, but it might be one of:

ppois(y,lambda=mu)  gives the probability of an observed value less than
or equal to y if the true mean is mu.

ppois(y,lambda=mu,lower.tail=FALSE)  give the probability of an observed
value greater than y if the true mean is mu.

qpois(alpha, lambda=mu,lower.tail=FALSE) gives the value of Y exceeded
with probability alpha.


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