The Excel GAMMAINV Function

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Related Function:

GAMMADIST

GAMMADIST and GAMMAINV

The Gammainv function is the inverse of the Excel Gammadist function.

I.e., for a supplied probability, the Gammainv function returns the value of x such that:

probability = Gammadist( x, alpha, beta, TRUE )

GAMMAINV and GAMMA.INV Functions

In Excel 2010, the Gammainv function has been replaced by the Gamma.Inv function, which has improved accuracy.

Although it has been replaced, the Gammainv 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 Gammainv function returns the inverse of the Gamma Distribution.

The syntax of the function is :

GAMMAINV( probability, alpha, beta )

where the function arguments are:

probability	-	The probability value for which you want to evaluate the inverse gamma distribution (must be between 0 & 1)
alpha	-	A parameter of the distribution
beta	-	A parameter of the distribution Note: if beta=0, then this specifies the Standard Gamma distribution

Gammainv Function Example

The charts below show the Cumulative Gamma Distribution and the Inverse Cumulative Gamma Distribution, with the parameter alpha set to 3 and the parameter beta set to 2.

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

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

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

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

If you want to calculate the value of the Inverse Cumulative Gamma Distribution for the probability 0.5, this can be done using the Excel Gammainv function, as follows:

=GAMMAINV( 0.5, 3, 2 )

This gives the result 5.348120627.

Further examples of the Excel Gammainv function can be found on the Microsoft Office website.

Gammainv Function Errors

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

Common Errors

#NUM!

Occurs if either:

	-	the supplied probability value is < 0 or ≥ 1
	-	the supplied alpha ≤ 0 or the supplied beta ≤ 0

#VALUE!

Occurs if any of the supplied arguments are not recognised as numeric values

#N/A

Occurs if Excel fails to converge to a solution after 100 iterations. Excel is unable to calculate the solution.

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