QQPLOT Statement: CAPABILITY Procedure
Overview: QQPLOT Statement
The QQPLOT statement creates a quantile-quantile plot (Q-Q plot), which compares ordered values of a variable with quantiles 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 Q-Q plot to determine how well a theoretical distribution models a set of measurements.
You can specify one of the following theoretical distributions with the QQPLOT 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 QQPLOT statement to do the following:
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specify or estimate parameters for the theoretical distribution
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display a reference line corresponding to specific location and scale parameters for the theoretical distribution
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request graphical enhancements
You can also create a comparative Q-Q plot by using the QQPLOT statement in conjunction with a CLASS statement.
You have three alternatives for producing Q-Q plots with the QQPLOT 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: Q-Q plots are similar to probability plots, which you can create with the PROBPLOT statement (see PROBPLOT Statement: CAPABILITY Procedure). Q-Q plots are preferable for graphical estimation of distribution parameters and capability indices, whereas probability plots are preferable for graphical estimation of percentiles.