Apple Numbers '08 - Manual (Page 261)

Chapter 12 Dictionary of Functions 261
PROB
The PROB function calculates the probability of a range of values if you know the
probabilities of the individual values.
PROB(values, probabilities, lower, [upper])
 values: A range of cells containing data values.
 probabilities: A range of cells containing the probability of each corresponding value
in values. The sum of the probabilities must add up to 1.
 lower: The lower limit of the range for which you want to find the probability.
 upper: Optional; the upper limit of the range for which you want to find the
probability. If omitted, PROB finds the probability of the single number specified for
lower.
Examples
If cells A1:E1 contain 1/1/2000, 1/1/2010, 1/1/1999, 0.1, 0.1:
PRICEMAT(A1, B1, C1, D1, E1) returns 95.
Examples
Given the following table:
PROB(A1:A10, C1:C10, 4, 6) returns 0.3, the probability that the value is 4, 5, or 6.
PROB(A1:A10, C1:C10, 5, 9) returns 0.5, the probability that the value is 5 or greater.
PROB(A1:A10, E1:E10, 4, 6) returns 0.59, the probability that the value is 4, 5, or 6.
PROB(A1:A10, E1:E10, 6) returns 0.21, the probability that the value is 6.