The Poisson distribution
is a discrete
probability distribution. It
is used when the sample size is unbounded
and it is not possible to specify the number
of occurrences, but it is possible to specify
the average number of occurrences:
d 
number of occurrences 
λ 
average number of occurrences 
The Poisson distribution is
often used in reliability engineering in
the form:
d 
number of occurrences 
λ 
average number of occurrences per
unit time 
t 
time period of interest 
Note that definition of 'λ
' has changed to the average number
of occurrences per unit time.
