Excel Function GAMMADIST

GAMMADIST(x, alpha, beta, cumulative)

Returns the probability of getting less than or equal to a particular value in a gamma distribution.

x	The number from the distribution.
alpha	The first parameter to the distribution.
beta	The second parameter to the distribution.
cumulative	The type of function to use: True = cumulative probability function (less than) False = probability mass function (equal to)

Remarks

* GAMMA.DIST was added in Excel 2010 to replace this function.
* The gamma distribution is a continuous distribution.
* If "x" < 0, then #NUM! is returned.
* If "x" is not numeric, then #VALUE! is returned.
* If "alpha" = 0, then #NUM! is returned.
* If "alpha" is not numeric, then #VALUE! is returned.
* If "beta" = 1, then the standard gamma distribution is used.
* If "beta" = 0, then #NUM! is returned.
* If "beta" is not numeric, then #VALUE! is returned.
* The "cumulative" argument is not optional.
* If "cumulative" = True, then the area under the curve to the left of "x" is returned. This uses the cumulative probability function.
* If "cumulative" = False, then the height of the curve at "x" is returned. This uses the probability mass function.
* This function is also known as the Erlang distribution, when "alpha" is a positive integer.
* In Excel 2010 the accuracy of this function was improved.
* For the Microsoft documentation refer to support.office.com

	A
1	=GAMMADIST(10,9,2,FALSE) = 0.033
2	=GAMMADIST(10,9,2,TRUE) = 0.068
3	=GAMMADIST(10,10,5,TRUE) = 0
4	=GAMMADIST(7,5,1,TRUE) = 1
5	=GAMMADIST(GAMMAINV(0.5,4,2),4,2,TRUE) = 1
6	=GAMMADIST(10,9,0,TRUE) = #NUM!
7	=GAMMADIST(10,-2,2,TRUE) = #NUM!
8	=GAMMADIST(-1,9,2,TRUE) = #NUM!
9	=GAMMADIST(-1,"some text",2,TRUE) = #VALUE!
10	=GAMMADIST(-1,9,2,"some text") = #VALUE!