[R] Normal and Poisson tail area expectations in R

[Charles C. Berry]

Ravi,

This looks simple to me.

km_G <- function(lambda,k)
 	lambda*ppois(k-1,lambda,lower=FALSE) -
 		k*ppois(k,lambda,lower=FALSE)

Am I confused here?

Chuck



On Wed, 13 Jun 2007, Ravi Varadhan wrote:

>
> More interesting is the Poisson convolution. I don't know if there is an
> analytic solution to this.  I looked at Jolley's "Summation of Series" and
> Abramowitz and Stegun, but no help there.  It seems that discrete FFT
> technique should work. Does anyone know the answer?
>
> Ravi.
> ----------------------------------------------------------------------------
> -------
>
> Ravi Varadhan, Ph.D.
>
> Assistant Professor, The Center on Aging and Health
>
> Division of Geriatric Medicine and Gerontology
>
> Johns Hopkins University
>
> Ph: (410) 502-2619
>
> Fax: (410) 614-9625
>
> Email: rvaradhan at jhmi.edu
>
> Webpage:  http://www.jhsph.edu/agingandhealth/People/Faculty/Varadhan.html
>
>
>
> ----------------------------------------------------------------------------
> --------
>
> -----Original Message-----
> From: r-help-bounces at stat.math.ethz.ch
> [mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of kavindra malik
> Sent: Wednesday, June 13, 2007 5:45 PM
> To: Charles C. Berry
> Cc: r-help at stat.math.ethz.ch
> Subject: Re: [R] Normal and Poisson tail area expectations in R
>
> Thank you very much. This solves the problem I was trying to solve. I am new
> to R and am learning. A great lesson in the power of R...
>
> "Charles C. Berry" <cberry at tajo.ucsd.edu> wrote: On Wed, 13 Jun 2007,
> kavindra malik wrote:
>
>> I am interested in R functions for the following integrals / sums
> (expressed best I can in text)  -
>>
>> Normal: G_u(k) =  Integration_{Lower limit=k}^{Upper limit=infinity} [(u
> -k) f(u) d(u)], where where u is N(0,1), and f(u) is the density function.
>>
>> Poisson: G(lambda,k) = Sum_{Lower limit=k}^{Upper limit=infinity} [(x-k)
> p(x, lambda)] where P(x,lambda) is the Poisson prob function with parameter
> lambda.
>>
>> The Normal expression is very commonly used in inventory management to
>> determine safety stocks (and its tabular values can be found in some
>> texts) - and I am also looking for Poisson and/or Gamma as that'd fit
>> the situation better.
>>
>> I am wondering if there are standard functions in R that might allow me to
> get these values, instead of needing to do the numerical integration, etc.
> myself.
>
> Not that I know of, but it is not difficult to do the integration:
>
>> k <- 1.1 # for example
>> integrate(function(x) (x-k)*dnorm(x),lower=k,upper=Inf)
> 0.06861951 with absolute error < 5.5e-07
>>
>
> see
>
>  ?integrate
>  ?qnorm
>  ?qpois
>  ?qgamma
>
>>                                                    Thank you very much.
>>
>>
>>
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>
> Charles C. Berry                            (858) 534-2098
>                                             Dept of Family/Preventive
> Medicine
> E mailto:cberry at tajo.ucsd.edu             UC San Diego
> http://famprevmed.ucsd.edu/faculty/cberry/  La Jolla, San Diego 92093-0901
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>

Charles C. Berry                            (858) 534-2098
                                             Dept of Family/Preventive Medicine
E mailto:cberry at tajo.ucsd.edu	            UC San Diego
http://famprevmed.ucsd.edu/faculty/cberry/  La Jolla, San Diego 92093-0901