QQPLOT Statement: CAPABILITY Procedure

Syntax: QQPLOT Statement

The syntax for the QQPLOT statement is as follows:

QQPLOT <variables> </ options> ;

You can specify the keyword QQ as an alias for QQPLOT, and you can use any number of QQPLOT statements in the CAPABILITY procedure. The components of the QQPLOT statement are described as follows.

variables

are the process variables for which to create Q-Q plots. If you specify a VAR statement, the variables must also be listed in the VAR statement. Otherwise, the variables can be any numeric variables in the input data set. If you do not specify a list of variables, then by default the procedure creates a Q-Q plot for each variable listed in the VAR statement, or for each numeric variable in the DATA= data set if you do not specify a VAR statement. For example, each of the following QQPLOT statements produces two Q-Q plots, one for length and one for width:

proc capability data=measures;
   var length width;
   qqplot;
run;

proc capability data=measures;
   qqplot length width;
run;

options

specify the theoretical distribution for the plot or add features to the plot. If you specify more than one variable, the options apply equally to each variable. Specify all options after the slash (/) in the QQPLOT statement. You can specify only one option naming the distribution in each QQPLOT statement, but you can specify any number of other options. The distributions available are the beta, exponential, gamma, Gumbel, lognormal, normal, generalized Pareto, power function, Rayleigh, two-parameter Weibull, and three-parameter Weibull. By default, the procedure produces a plot for the normal distribution.

In the following example, the NORMAL option requests a normal Q-Q plot for each variable. The MU= and SIGMA= normal-options request a distribution reference line with intercept 10 and slope 0.3 for each plot, corresponding to a normal distribution with mean $\mu =10$ and standard deviation $\sigma =0.3$ . The SQUARE option displays the plot in a square frame, and the CTEXT= option specifies the text color.

proc capability data=measures;
   qqplot length1 length2 / normal(mu=10 sigma=0.3)
                            square
                            ctext=blue;
run;