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Topic: How to simulate a Poisson sample?
Replies: 1   Last Post: Oct 6, 2013 5:24 AM

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Luis A. Afonso

Posts: 4,758
From: LIsbon (Portugal)
Registered: 2/16/05
How to simulate a Poisson sample?
Posted: Oct 5, 2013 4:50 AM
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How to simulate a Poisson sample?

The discrete r.v. is said to have a Poisson Distribution with a positive real parameter lambda if the probability mass function is

P(X=x) =
= 1/x! * [exp(-lambda) * lambda^x]___(1)
X=0, 1, 2, 3, ...
lambda = not zero real positive.

In order to simulate the r.v. Let be a random number u. As long as the C(x) - Cumulative Distribution Function - right bounds are not larger than u we continue the comparison procedure: The X=x is got when we find Sum [P(x)]>u.
Note that a recursive algorithm allows to calculate the probability of no matter x:
P(x+1)= P(x) * lambda/(x+1), P(x=0) = exp(-lambda)

For example u= 0.9920 and lambda=1 the cumulative distribution C(x) is

____C(0)= 0.3679 = P(0) = exp(-1)
____C(1)= 0.7358
____C(2)= 0.9197
____C(3)= 0.9810_____0.9920
____C(4)= 0.9963
. . . . . . . And we get x=3.

This method is both rigorous and general: it works for any discrete Distribution to which one can obtain the Cumulative Distribution inverse, x= f(CDF) = f(RND). If the r.v. X is continuous, given RND, to try, step h, successively, x= x0 + j*h (x0 negative say -10000 , h=1), just till CDF(x1) > RND, to go 2 steps behind, CDF(x´= x1-2*h), put h1=h/10 and repeat the procedure till CDF (x2) > RND, x3=x2 - 2*h1, and so on till CDF(x) = RND with very good approximation, for example hn = .000001.

Example, Phi the cumulative normal, 0< p<1 , B, C, known,

CDF = p* Phi(x/B) + (1-p) * Phi(x/C)

Luis A. Afonso

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