POISSON function returns the Poisson distribution. @x is the number of events, @mean is the expected numeric value @cumulative describes whether to return the sum of the poisson function from 0 to @x.
If @x is a non-integer it is truncated. If @x <= 0 POISSON returns #NUM! error. If @mean <= 0 POISSON returns the #NUM! error.
This function is Excel compatible.