The formula for the Poisson distribution (which can be derived
from the binomial distribution  see Comparison of
distributions) is
where r is the number observed and m is the
mean value.
Choose
values of m to see the change in the distribution shape
P(r)
Note how P(r) becomes more symmetrical as m increases (»1).
variance
of the Poisson distribution s^{2}= m, standard
deviation s= .
