Question about "gamcdf" code

The scale parameter is 2 and shape parameter is 1. I want to use "gamcdf" code for 75th percentile value, then do I have to use gamcdf(75,2,1) or gamcdf(0.75,2,1) ?

Answer 1

[Jeff Miller]

To get the 75th percentile value, you need to use gaminv() rather than gamcdf. Also, the shape parameter comes first and scale second. So, you want

gaminv(0.75,1,2)
% ans = 2.7726

With a shape parameter of 1, this is the same as the exponential distribution:

expinv(0.75,2)
% ans = 2.7726