GAMMADIST Function

Basic Description

The Excel Gammadist function returns the Gamma Distribution, which is frequently used to provide probabilities for values that may have a skewed distribution, such as queuing analysis. The user can specify whether the cumulative distribution or the probability density function should be used.

Syntax: GAMMADIST( x, alpha, beta, cumulative )

where the function arguments listed in the following table:

x The value at which you want to evaluate the function (must be ≥ 0)
alpha A parameter of the distribution
beta A parameter of the distribution
Note: if beta=0, then this specifies the Standard Gamma distribution
cumulative A logical argument that specifies the type of gamma distribution to be calculated. This can have the value TRUE or FALSE, meaning:

TRUE calculate the cumulative distribution function
FALSE calculate the probability density function

 

Gammadist Function Examples

Example 1 – Probability Density Function

The chart on the right shows the probability density function for the gamma distribution with the parameter α = 3 and the parameter β = 2.

If you want to calculate the value of this function at the value x = 6, this can be done by the Excel Gammadist function, as follows:

=GAMMADIST( 6, 3, 2, FALSE )

This gives the result 0.112020904.

Plot of the Probability Density Gamma Distribution with α = 3 and β = 2

Gamma Probability Density Distribution with α=3 & β=2
Example 2 – Cumulative Distribution Function

The chart on the right shows the Cumulative Gamma Distribution with the parameter α = 3 and the parameter β = 2.

If you want to calculate the value of this function at the value x = 6, this can be done by the Excel Gammadist function, as follows:

=GAMMADIST( 6, 3, 2, TRUE )

This gives the result 0.576809919.

Plot of the Cumulative Gamma Distribution Function with α = 3 and β = 2

Cumulative Gamma Distribution Function with α=3 & β=2

 

Gammadist Function Errors

If you get an error from the Excel Gammadist function this is likely to be one of the following :

#NUM! Occurs if either the supplied value of x is < 0 or the supplied alpha ≤ 0 or the supplied beta ≤ 0
#VALUE! Occurs if any of the supplied arguments, x, alpha or beta are not recognised as numeric values

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