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The Excel GAMMADIST Function
GAMMADIST and GAMMA.DIST Functions
In Excel 2010, the Gammadist function has been replaced by the
Gamma.Dist function,
which has improved accuracy.
Although it has been replaced, the Gammadist function is still available in Excel 2010 (stored in the list
of compatibility functions), to allow compatibility with earlier versions of Excel.
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.
The syntax of the Gammadist function is :
GAMMADIST( x, alpha, beta, cumulative )
where the function arguments are:
| x |
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The value at which you want to evaluate the function (must be ≥ 0)
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| alpha |
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A parameter of the distribution
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| beta |
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A parameter of the distribution
Note: if beta=0, then this specifies the Standard Gamma distribution
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| 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 |
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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.
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Gamma Probability Density Distribution with α=3 & β=2
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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.
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Cumulative Gamma Distribution Function with α=3 & β=2
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Further examples of the Excel Gammadist function can be found on the
Microsoft Office website.
Gammadist Function Errors
If you get an error from the Excel Gammadist function this is likely to be one of the following :
Common Errors
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#NUM!
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Occurs if either:
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the supplied value of x is < 0
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the supplied alpha ≤ 0 or the supplied beta ≤ 0
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#VALUE!
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Occurs if any of the supplied arguments, x, alpha or beta are not recognised as numeric values
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