Probability of events for a Poisson distribution

An event can occur 0, 1, 2, … times in an interval. The average number of events in an interval is designated ${\displaystyle \lambda }$ (lambda). Lambda is the event rate, also called the rate parameter. The probability of observing k events in an interval is given by the equation

${\displaystyle P(k{\text{ events in interval}})=e^{-\lambda }{\frac {\lambda ^{k}}{k!}}}$

where

${\displaystyle \lambda }$ is the average number of events per interval

e is the number 2.71828... (Euler's number) the base of the natural logarithms

k takes values 0, 1, 2, …

k! = k × (k − 1) × (k − 2) × … × 2 × 1 is the factorial of k.

For example: Salesman sales, in average, 5 products per week. The chance he will sale, in a given week 10 products is 1.8133%

(λ=2; k=10)