gaminv
Inverse of the Gamma cumulative distribution function (iCDF).
For each element of p, compute the quantile (the inverse of the CDF) of the Gamma distribution with shape parameter a and scale parameter b. The size of x is the common size of p, a, and b. A scalar input functions as a constant matrix of the same size as the other inputs.
OCTAVE/MATLAB use the alternative parameterization given by the pair , i.e. shape a and scale b. In Wikipedia, the two common parameterizations use the pairs , as shape and scale, and , as shape and rate, respectively. The parameter names a and b used here (for MATLAB compatibility) correspond to the parameter notation instead of the as reported in Wikipedia.
Further information about the Gamma distribution can be found at https://en.wikipedia.org/wiki/Gamma_distribution
See also: gamcdf, gampdf, gamrnd, gamfit, gamlike, gamstat
Source Code: gaminv
## Plot various iCDFs from the Gamma distribution
p = 0.001:0.001:0.999;
x1 = gaminv (p, 1, 2);
x2 = gaminv (p, 2, 2);
x3 = gaminv (p, 3, 2);
x4 = gaminv (p, 5, 1);
x5 = gaminv (p, 9, 0.5);
x6 = gaminv (p, 7.5, 1);
x7 = gaminv (p, 0.5, 1);
plot (p, x1, "-r", p, x2, "-g", p, x3, "-y", p, x4, "-m", ...
p, x5, "-k", p, x6, "-b", p, x7, "-c")
ylim ([0, 20])
grid on
legend ({"α = 1, β = 2", "α = 2, β = 2", "α = 3, β = 2", ...
"α = 5, β = 1", "α = 9, β = 0.5", "α = 7.5, β = 1", ...
"α = 0.5, β = 1"}, "location", "northwest")
title ("Gamma iCDF")
xlabel ("probability")
ylabel ("x")
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