PROBPLOT Statement: CAPABILITY Procedure

Overview: PROBPLOT Statement

The PROBPLOT statement creates a probability plot, which compares ordered values of a variable with percentiles of a specified theoretical distribution such as the normal. If the data distribution matches the theoretical distribution, the points on the plot form a linear pattern. Thus, you can use a probability plot to determine how well a theoretical distribution models a set of measurements.

You can specify one of the following theoretical distributions with the PROBPLOT statement:

  • beta

  • exponential

  • gamma

  • Gumbel

  • three-parameter lognormal

  • normal

  • generalized Pareto

  • power function

  • Rayleigh

  • two-parameter Weibull

  • three-parameter Weibull

You can use options in the PROBPLOT statement to do the following:

  • specify or estimate shape parameters for the theoretical distribution

  • display a reference line corresponding to specified or estimated location and scale parameters for the theoretical distribution

  • request graphical enhancements

You can also create a comparative probability plot by using the PROBPLOT statement in conjunction with a CLASS statement.

You have three alternatives for producing probability plots the PROBPLOT statement:

  • ODS Graphics output is produced if ODS Graphics is enabled, for example by specifying the ODS GRAPHICS ON statement prior to the PROC statement.

  • Otherwise, traditional graphics are produced by default if SAS/GRAPH® is licensed.

  • Legacy line printer charts are produced when you specify the LINEPRINTER option in the PROC statement.

See Chapter 3: SAS/QC Graphics, for more information about producing these different kinds of graphs.

Note: Probability plots are similar to Q-Q plots, which you can create with the QQPLOT statement (see QQPLOT Statement: CAPABILITY Procedure). Probability plots are preferable for graphical estimation of percentiles, whereas Q-Q plots are preferable for graphical estimation of distribution parameters and capability indices.