QQPLOT Statement: CAPABILITY Procedure

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:

• 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 QQPLOT statement to do the following:

• specify or estimate parameters for the theoretical distribution

• display a reference line corresponding to specific location and scale parameters for the theoretical distribution

• 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:

• 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: 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.