Poisson Distribution:
Poisson distribution is a discrete distribution, completely characterized by one parameter 
:
(where k! = 1 x 2 x ... x k).
Both the mean and the variance of Poisson distribution are equal to 
.
Poisson distribution describes the number of random events on an interval in space and time. If t is duration (or length) of the interval, and 
is the rate of the events (i.e. the average number of events per a unit interval), then 
. See also Poisson process for more precise definition of "random events".
For large enough values of the parameter 
, Poisson distribution is close to a normal distribution with the mean and the variance equal to 
.