# Mode of the Poisson Distribution

1. Nov 28, 2012

### Ryuky

Hello People,

Lately, I am doing an investigation on Stirling's formula and its applications. So I thought I could use it to prove that the mode of the Poisson model is approximately equal to the mean. Of course, you do that by considering the curve that is formed by connecting the points of the probabilities of occurrence and the different values of the discrete random variable. Then you differentiate the p.d.f. where for x! you use Stirling's formula x!~sqrt{2pi*x}x^x*e^-x. The result is lnλ-1/(2x)-lnx whose roots cannot be found analytically, but by iterative methods we find that as λ is larger and larger, the mode~mean.

Problem is, I found the following paper online, which seems to be the solution from a Harvard's undergraduate problem set.

It reads "You can also show this by taking the derivative of eq. (2), with Stirling’s expression in place of the x!. Furthermore, you can show that x = a[=λ in my case]-1/2 leads to a maximum P(x) value of Pmax~1/sqrt{2pa}."

Does this puzzle you as much as it puzzles me? My main concern is over the "=" sign: how does this hold? The derivative=0 equation cannot have such an exact solution. Furthermore at such x, how does P(X=a-1/2) give 1/sqrt{2pia}?

Am I (and my professor) missing something rather obvious or is the solution wrong?

Discuss!

Last edited: Nov 28, 2012