PROBPLOT Statement: CAPABILITY Procedure
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:
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beta
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exponential
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gamma
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Gumbel
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three-parameter lognormal
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normal
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generalized Pareto
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power function
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Rayleigh
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two-parameter Weibull
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three-parameter Weibull
You can use options in the PROBPLOT statement to do the following:
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specify or estimate shape parameters for the theoretical distribution
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display a reference line corresponding to specified or estimated location and scale parameters for the theoretical distribution
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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:
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ODS Graphics output is produced if ODS Graphics is enabled, for example by specifying the ODS GRAPHICS ON statement prior to the PROC statement.
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Otherwise, traditional graphics are produced by default if SAS/GRAPH® is licensed.
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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.