Specifying the Computation of the Moving Range

[See SHWIR1 in the SAS/QC Sample Library]By default, the IRCHART statement uses two consecutive measurements to calculate moving ranges. However, you can specify a different number of measurements to use, as illustrated by the following statements:

ods graphics on;
title 'Specifying the Computation of the Moving Range';
proc shewhart data=Jets;
   irchart Diam*Engine / limitn=3 odstitle=title;
run;
ods graphics off;

The ODS GRAPHICS ON statement specified before the PROC SHEWHART statement enables ODS Graphics, so the charts are created by using ODS Graphics instead of traditional graphics. The LIMITN= option specifies the number of consecutive measurements used to compute the moving ranges. The resulting charts are shown in Figure 13.10.10.

Output 13.10.10 Computing Moving Ranges from Three Consecutive  Measurements (ODS Graphics)

Note that the LIMITN= value is displayed in the legend above the control limit labels. The charts indicate that the process is in control, since all the points lie within the control limits.