Returns the gamma distribution. You can use this function to study variables that may have a skewed distribution. The gamma distribution is commonly used in queuing analysis.
Syntax
GAMMADIST(x,alpha,beta,cumulative)
X is the value at which you want to evaluate the distribution.
Alpha is a parameter to the distribution.
Beta is a parameter to the distribution. If beta = 1, GAMMADIST returns the standard gamma distribution.
Cumulative is a logical value that determines the form of the function. If cumulative is TRUE, GAMMADIST returns the cumulative distribution function; if FALSE, it returns the probability mass function.
Remarks

If x, alpha, or beta is nonnumeric, GAMMADIST returns the #VALUE! error value.

If x < 0, GAMMADIST returns the #NUM! error value.

If alpha ≤ 0 or if beta ≤ 0, GAMMADIST returns the #NUM! error value.

The equation for the gamma distribution is:
The standard gamma distribution is:

When alpha = 1, GAMMADIST returns the exponential distribution with:

For a positive integer n, when alpha = n/2, beta = 2, and cumulative = TRUE, GAMMADIST returns (1  CHIDIST(x)) with n degrees of freedom.

When alpha is a positive integer, GAMMADIST is also known as the Erlang distribution.
Examples
X 
Alpha 
Beta 
Formula 
Description (Result) 
10 
9 
2 
=GAMMADIST([X],[Alpha],[Beta],FALSE) 
Probability gamma distribution of the arguments (0.032639) 
10 
9 
2 
=GAMMADIST([X],[Alpha],[Beta],TRUE) 
Cumulative gamma distribution of the arguments (0.068094) 