Poisson limit theorem

"Poisson theorem" redirects here. For the "Poisson's theorem" in Hamiltonian mechanics, see Poisson bracket § Constants of motion.

The law of rare events or Poisson limit theorem gives a Poisson approximation to the binomial distribution, under certain conditions.[1] The theorem was named after Siméon Denis Poisson (17811840).

Statement

If

n \rightarrow \infty, p \rightarrow 0, such that np \rightarrow \lambda

then

\frac{n!}{(n-k)!k!} p^k (1-p)^{n-k} \rightarrow e^{-\lambda}\frac{\lambda^k}{k!}.

Example

Suppose that in an interval [0, 1000], 500 points are placed randomly. Now what is the number of points that will be placed in [0, 10]?

The probabilistically precise way of describing the number of points in the sub-interval would be to describe it as a binomial distribution p_n(k).

If we look here, the probability (that a random point will be placed in the sub-interval) is p = 10/1000 = 0.01. Here n=500 so np=5.

The probability that k points lie in the sub-interval is

p_n(k)=\frac{n!}{(n-k)!k!} p^k (1-p)^{n-k}.

where: p is the probability of falling with in the interval. n!/(k! \cdot (n-k)!) gives the number of ways in which k elements can be selected. p^k gives the probability of the k elements falling in the interval. (1-p)^{n-k} counts the probability that {n-k} elements fall outside of the interval

But using the Poisson Theorem we can approximate it as

e^{-\lambda}\frac{\lambda^k}{k!} = e^{-5}\frac{5^k}{k!}.

Proofs

According to factorial's rate of growth, we replace factorials of large numbers with approximations:

\frac{n!}{(n-k)!k!} p^k (1-p)^{n-k} \rightarrow \frac{ \sqrt{2\pi n}\left(\frac{n}{e}\right)^n}{ \sqrt{2\pi \left(n-k\right)}\left(\frac{n-k}{e}\right)^{n-k}k!} p^k (1-p)^{n-k}.

After simplifying the fraction:

\frac{ \sqrt{2\pi n}\left(\frac{n}{e}\right)^n}{ \sqrt{2\pi \left(n-k\right)}\left(\frac{n-k}{e}\right)^{n-k}k!} p^k (1-p)^{n-k}\rightarrow \frac{ \sqrt{n}n^np^k (1-p)^{n-k}}{ \sqrt{n-k}\left(n-k\right)^{n-k}e^kk!}\rightarrow \frac{n^np^k (1-p)^{n-k}}{\left(n-k\right)^{n-k}e^kk!}.

After using the condition np \rightarrow \lambda:

 \frac{n^np^k (1-p)^{n-k}}{\left(n-k\right)^{n-k}e^kk!} \rightarrow \frac{n^k\left(\frac{\lambda}{n}\right)^k (1-\frac{\lambda}{n})^{n-k}}{\left(1-\frac{k}{n}\right)^{n-k}e^kk!}=\frac{\lambda^k \left(1-\frac{\lambda}{n}\right)^{n-k}}{\left(1-\frac{k}{n}\right)^{n-k}e^kk!}\rightarrow\frac{\lambda^k \left(1-\frac{\lambda}{n}\right)^{n}}{\left(1-\frac{k}{n}\right)^{n}e^kk!}

As n\rightarrow \infty, \left(1+\frac{x}{n}\right)^n \rightarrow e^x so:

\frac{\lambda^k \left(1-\frac{\lambda}{n}\right)^{n}}{\left(1-\frac{k}{n}\right)^{n}e^kk!}\rightarrow\frac{\lambda^k e^{-\lambda}}{e^{-k}e^kk!}=\frac{\lambda^k e^{-\lambda}}{k!}

Q.E.D.

Alternative Proof

If we make the stronger assumption  np=\lambda (rather than  np\rightarrow \lambda ) then a simpler proof is possible without needing approximations for the factorials. Since np=\lambda, we can rewrite p=\lambda / n. We now have:

\lim_{n\to\infty}\frac{n!}{(n-k)!k!} \left(\frac{\lambda}{n}\right)^k  \left(1- \frac{\lambda}{n}\right)^{n-k}
= \lim_{n\to\infty}\frac{n(n-1)(n-2)\dots(n-k+1)}{k!} \frac{\lambda^k}{n^k}  \left(1- \frac{\lambda}{n}\right)^{n-k}

Taking each of these terms in sequence, n(n-1)(n-2)\dots(n-k+1)=n^k+O\left(n^{k-1}\right), meaning that \lim_{n\to\infty} \frac{n(n-1)(n-2)\dots(n-k+1)}{n^k k!}= \frac{1}{k!}.

Now \left(1- \frac{\lambda}{n}\right)^{n-k}=\left(1- \frac{\lambda}{n}\right)^{n}\left(1- \frac{\lambda}{n}\right)^{-k}. The first portion of this converges to e^{-\lambda}, and the second portion goes to 1, as \lim_{n\to\infty}\left(1- \frac{\lambda}{n}\right)^{-k}=\lim_{n\to\infty}\left(1- 0\right)^{-k}=1

This leaves us with \frac{1}{k!}\lambda^k e^{-\lambda}. Q.E.D.

Ordinary Generating Functions

It is also possible to demonstrate the theorem through the use of Ordinary Generating Functions (OGF). Indeed, the OGF of the binomial distribution is


  G_\mathrm{bin}(x;p,N) 
    \equiv \sum_{k=0}^{N} \left[ \binom{N}{k} p^k (1-p)^{N-k} \right] x^k
    = \Big[ 1 + (x-1)p \Big]^{N}

by virtue of the Binomial Theorem. Taking the limit N \rightarrow \infty while keeping the product pN\equiv\lambda constant, we find

 
  \lim_{N\rightarrow\infty} G_\mathrm{bin}(x;p,N)
    = \lim_{N\rightarrow\infty} \Big[ 1 + \frac{\lambda(x-1)}{N} \Big]^{N} 
    = \mathrm{e}^{\lambda(x-1)}
    = \sum_{k=0}^{\infty} \left[ \frac{\mathrm{e}^{-\lambda}\lambda^k}{k!} \right] x^k

which is the OGF for the Poisson distribution. (The second equality holds due to the definition of the Exponential function.)

See also

References

  1. Papoulis, Pillai, Probability, Random Variables, and Stochastic Processes, 4th Edition
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