You can model two additional probability distributions, the chi-square and Erlang distributions, by adjusting the parameters entered in the Gamma Distribution dialog. To model the chi-square distributions with parameters N and S, where N = number of degrees of freedom and S = scale, set the parameters as follows:
The chi-square distribution is the sum of the squares of N normal variates.
The Erlang distribution is identical to the gamma distribution, except the shape parameter is restricted to integer values. Mathematically, the Erlang distribution is a summation of N exponential distributions.