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cdfgam_p

Calculates the integral of a cumulative gamma distribution function.

Prototype

	function cdfgam_p (
		x      : numeric,  
		shape  : numeric,  
		scale  : numeric   
	)

	return_val  :  numeric

Arguments

A multi-dimensional array or scalar value equal to upper limit of integration of the gamma density. (x ≥ 0.)

shape

A multi-dimensional array or scalar value equal to the shape parameter of the gamma density (shape > 0.0)

scale

A multi-dimensional array or scalar value equal to the scale parameter of the gamma density (scale > 0.0). In some instances, it may be appropriate to use 1/scale as the input. See the Description section below for additional notes on usage.

Return value

A array of the same size as x. Double if any of the input arguments are double, float otherwise.

Description

Calculates the integral of a cumulative gamma distribution function.

The source code is from source routine "CDFGAM" from DCDFLIB (Double precision Cumulative Distribution Function LIBrary)

NCL's cdfgam_p and the R function pgamma use the same convention for the scale and shape arguments. To compare with results from Excel's GAMMADIST function and Matlab's gamcdf function, the scale argument may have to be inverted.

Details: The issue comes from the convention in the parameterization.

Some languages use as input for the two parameters that characterize a Gamma distribution a and b such that a/b is the mean of the distribution and a/b^2 is the variance; other languages use a and b such that a*b is the mean and a*b^2 is the variance. NOTE: When b, the scale factor is one (1.0), there will be no difference in the returned values.

Excel's GAMMADIST uses a parameterization different from NCL's cdfgam_p and R's pgamma. Again, there is no difference if the scale is one (1.0).

Examples

Example 1

     x = 6.29579
     shape = 3.0
     scale = 1.0   ; returned value is sensitive to the scale value

     P = cdfgam_p(x,shape,scale)
     print("P="+P)               ; P = 0.95

Example 2: Compare with R function pgamma.


%> cat gamma.R
pgamma(98, 100, 1)
pgamma(21, 100, 5)
pgamma(2, 100, 69)

---
%> R -q --vanilla < gamma.R
      > pgamma(98, 100, 1)
      [1] 0.4333105
      > pgamma(21, 100, 5)
      [1] 0.7002453
      > pgamma(2, 100, 69)
      [1] 0.9997046
---

%> cat gamma.ncl
   print("cdfgam_p(98, 100, 1) = "+cdfgam_p(98, 100, 1))
   print("cdfgam_p(21, 100, 5) = "+cdfgam_p(21, 100, 5))
   print("cdfgam_p( 2, 100,69) = "+cdfgam_p( 2, 100,69))

%> ncl gamma.ncl

(0)    cdfgam_p(98, 100, 1) = 0.433311
(0)    cdfgam_p(21, 100, 5) = 0.700245
(0)    cdfgam_p( 2, 100,69) = 0.999705

Example 3: Compare with Excel's GAMMADIST function [Matlab's gamcdf] with NCL's cdfgam_p and R's pgamma function. Note that the 'scale' argument must be inverted to match the Excel/Matlab result.