Plot: PP

The options described here are used to modify the existing Probability-Probability plot using a variety of distributions. Various options are available for specifying the parameters of the distribution that you select for your plot. These options are available in the PP dialog as well as on the PP tab of the Plot Layout dialog and via the Graph Options dialog. To learn more about the usefulness and interpretation of Probability-Probability plots, refer to this topic in the Glossary.

Plot. Use the Plot drop-down box to select the plot to which you want to make changes. If you have several plots in one graph, you assign names to each plot using the Name field on the Plot: General tab.

Distribution. The Distribution group box provides a list of the theoretical distributions from which you can select a distribution for the Probability-Probability plot (Normal, Beta, Exponential, Extreme, Gamma, Lognormal, Rayleigh, Weibull). When you select a distribution from this list, other options for specifying the parameters will be available, depending upon the distribution.

Do not assign average ranks to tied observations. This check box is selected by default, which enables you to see all data points, even the ties. When this check box is not selected, the "tied" points will overlap.

Parameters. STATISTICA offers three ways to supply the parameter(s) required for fitting the Beta, Gamma, Lognormal, or Weibull distribution in the Probability-Probability plot and are described below.

1. If you want to use the default values, then first select the desired distribution and then clear the Compute from data check box. STATISTICA will fit the standardized distribution using the default values of the parameters which depends on the distribution you selected (Mu = 1, Sigma = 1 for the Normal distribution; Shape 1 = 1, Shape 2 = 1 for the Beta distribution; Scale(1/lambda ) = 1 for the Exponential distribution; Threshold = 1, Scale = 0 for the Extreme distribution;  Shape = 1, Scale = 1 for the Gamma distribution; Shape = 1, Scale= 1 for the Lognormal distribution; Scale = 1 for the Rayleigh distribution; and Shape = 1 Scale = 1 for the Weibull distribution).

2. If you want to supply your own values for the shape parameter(s), then you first need to clear the Compute from data check box. You can then specify the desired values of the parameters for the selected distribution in the appropriate edit fields (see above).

3. If you want to compute (estimate) the shape parameters, then you need to select the Compute from data check box. In that case, you also need to enter the values in the Threshold and Scale fields, if the selected distribution is Beta or Extreme. If the selected distribution is Exponential, Gamma, Lognormal, Rayleigh, or Weibull, then only a threshold value needs to be entered in the Threshold field. The shape parameter(s) will in that case be estimated either by using the maximum likelihood method (see below) or by matching moments approximation. Note that the maximum likelihood method of estimation is available only if the selected distribution is Beta, Gamma, or Weibull.  

Use Max. Likelihood. Select the Use Max. Likelihood check box to use the maximum likelihood method to estimate the parameters (see Evans, Hastings, & Peacock, 1993, for details). If this check box is cleared, then the parameters are estimated by the method of matching moments. Note that the Use Max. Likelihood check box will only be available if the selected distribution is Beta, Gamma, or Weibull and the Compute from data check box is selected.