Apple iWork '09 - Manual (Page 282)

POISSON
The POISSON function returns the probability that a specific number of events will
occur using the Poisson distribution.
POISSON(events, average, form-type)
 events: The number of events (arrivals) for which you want to calculate the
probability. events is a number value.
 average: The average of the distribution. average is a number value representing
the known average (arithmetic mean) rate at which events occur.
 form-type: A value that indicates which form of the exponential function to
provide.
cumulative form (TRUE or 1): Return the value of the cumulative distribution
function form (that the specified number or fewer successes or events will occur).
probability mass form (FALSE or 0): Return the value of the probability mass
function form (that there are exactly the specified number of successes or events).
Example
For a mean of 10 and an arrival rate of 8:
=POISSON(8, 10, FALSE) returns 0.112599.
Related Topics
For related functions and additional information, see:
“EXPONDIST” on page 253
“Listing of Statistical Functions” on page 225
“Value Types” on page 36
“The Elements of Formulas” on page 15
“Using the Keyboard and Mouse to Create and Edit Formulas” on page 26
“Pasting from Examples in Help” on page 41
PROB
The PROB function returns the probability of a range of values if you know the
probabilities of the individual values.
PROB(num-set, probability-values, lower, upper)
 num-set: A collection of numbers. num-set is a collection containing number values.
282 Chapter 10 Statistical Functions