QQPLOT Statement: CAPABILITY Procedure

Adding a Distribution Reference Line

Note: See Creating Normal Q-Q Plots in the SAS/QC Sample Library.

In a normal Q-Q plot, the normal distribution with mean $\mu _0$ and standard deviation $\sigma _0$ is represented by a reference line with intercept $\mu _0$ and slope $\sigma _0$ . The following statements reproduce the Q-Q plot in Figure 5.39, adding the line for which $\mu _0$ and $\sigma _0$ are estimated by the sample mean and standard deviation:

title 'Normal Quantile-Quantile Plot for Hole Distance';
proc capability data=Sheets noprint;
   spec lsl=9.5 usl=10.5;
   qqplot Distance / normal(mu=est sigma=est)
                     square
                     nospeclegend
                     odstitle=title;
run;

The plot is displayed in Figure 5.40.

Figure 5.40: Adding a Distribution Reference Line to a Q-Q Plot

Specifying MU=EST and SIGMA=EST with the NORMAL option requests the reference line (alternatively, you can specify numeric values for $\mu _0$ and $\sigma _0$ with the MU= and SIGMA= options). The COLOR= and L= options specify the color of the line and the line type. The SQUARE option displays the plot in a square format, and the NOSPECLEGEND option suppresses the legend for the specification lines.