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

• three-parameter lognormal

• normal

• 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:

• Traditional graphics are produced by default.

• ODS Graphics output is produced when you specify the ODS GRAPHICS statement prior to the PROC CAPABILITY statement.

• Legacy line printer plots are produced when you specify the LINEPRINTER option in the PROC CAPABILITY 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). Probability plots are preferable for graphical estimation of percentiles, whereas Q-Q plots are preferable for graphical estimation of distribution parameters and capability indices.