The CDF of the Gamma distribution in GAMS

For a model (1) I needed to calculate the cumulative distribution function \(F(x)\) of the Gamma distribution. There is no uniform consensus:  the Gamma distribution has different parametrizations. I use the \(k, \theta\) parametrization from (2). This would yield (3):

\[F(x) =  \gamma( \frac{x}{\theta},k)\]

where \(\gamma(x,a)\) is the incomplete Gamma function defined by:

\[\gamma(x,a) = \frac{1}{\Gamma(a)} \int_0^x t^{a-1}e^{-t} dt\]

The function \(\Gamma(x)\) is the (complete) Gamma function:

\[\Gamma(x) = \int_0^{\infty} t^{x-1} e^{-t} dt \]

Unfortunately there a few alternative definitions of the incomplete Gamma function. In (2) the following definition is used:

\[\gamma(s,x) = \int_0^x t^{s-1} e^{-t} dt\]

So watch out: different definitions are being used.

As  a result we can implement the CDF of the Gamma distribution with parameter \(k\) and \(\theta\) as follows (2):

GammaReg(x/theta,k)

There is also a more obscure way to calculate this using an extrinsic function. The syntax is unfortunately needlessly complicated and the documentation is really, really bad (4).

scalars   k / 0.363 /   theta / 27.863 /   x / 1.5 /   p ; p = gammareg(x/theta,k); display p; $FuncLibIn stodclib stodclib function cdfGamma / stodclib.cdfGamma /; p = cdfGamma(x,k,theta); display p;

References

1. Modeling Flood Damages, http://yetanothermathprogrammingconsultant.blogspot.com/2017/07/modeling-flood-damages.html
2. Gamma Distribution, https://en.wikipedia.org/wiki/Gamma_distribution
3. New Special Functions in GAMS, http://www.amsterdamoptimization.com/pdf/specfun.pdf
4. Stochastic Library, https://www.gams.com/latest/docs/userguides/userguide/_u_g__extrinsic_functions.html#UG_ExtrinsicFunctions_StochasticLibrary