Gamma Distribution for Probability-Probability Plots
The Gamma distribution has the probability density function:
Compute from data
When you clear this check box (on the Probability-Probability Plots Advanced tab), you then need to specify the Shape and Scale parameters (c and b, respectively) as well as the Threshold parameter q. When you select the check box and specify the Threshold parameter q, Statistica estimates both the Shape and Scale parameters (c and b, respectively) from the data.
In general, if the observed points follow the Gamma distribution with the respective parameters, then they will fall onto the straight line in the P-P plot.
Use Max. Likelihood
The Use Max. Likelihood check box is displayed when you select the Gamma distribution on the Probability-Probability Plots - Advanced tab.