Specifying the Computation of the Moving Range

IRCHART Statement

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:

   title 'Specifying the Computation of the Moving Range';
   symbol v=dot c=yellow;
   proc shewhart data=jets;
      irchart diam*engine / limitn   = 3
                            cframe   = vigb
                            cinfill  = vlib
                            cconnect = yellow
                            coutfill = red;
   run;

The LIMITN= option specifies the number of consecutive measurements used to compute the moving ranges. The resulting charts are shown in Figure 34.8.

Figure 34.8: Computing Moving Ranges from Three Consecutive Measurements

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.

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