XRCHART Statement: SHEWHART Procedure

Reading Preestablished Control Limits

Note: See Mean and Range (X-Bar and R) Charts in the SAS/QC Sample Library.

In a previous example, the OUTLIMITS= data set saved control limits computed from the measurements in Wafers. This example shows how these limits can be applied to new data provided in the following data set: 

data Wafers2;
   input Batch @;
   do i=1 to 5;
      input Diameter @;
      output;
   end;
   drop i;
   datalines;
26 34.99 34.99 35.00 34.99 35.00
27 34.99 35.01 34.98 34.98 34.97
28 35.00 34.99 34.99 34.99 35.01
29 34.98 34.96 34.98 34.98 34.99
30 34.98 35.00 34.98 34.98 34.99
31 35.00 35.00 34.99 35.01 35.01
32 35.00 34.99 34.98 34.98 35.00
33 34.98 35.00 34.99 35.00 35.01
34 35.00 34.97 35.00 34.99 35.01
35 34.99 34.99 34.98 34.99 34.98
36 35.01 34.98 34.99 34.99 35.00
37 35.01 34.99 34.97 34.98 35.00
38 34.98 34.99 35.00 34.98 35.00
39 34.99 34.99 34.99 34.99 35.01
40 34.99 35.01 35.00 35.01 34.99
41 34.99 35.00 34.99 34.98 34.99
42 35.00 34.99 34.98 34.99 35.00
43 34.99 34.98 34.98 34.99 34.99
44 35.00 35.00 34.98 35.00 34.99
45 34.99 34.99 35.00 34.99 34.99
;

The following statements use the control limits in Waferlim to create $\bar{X}$ and R charts for the data in Wafers2: 

ods graphics on;
proc shewhart data=Wafers2 limits=Waferlim;
   xrchart Diameter*Batch;
run;

The ODS GRAPHICS ON statement specifies that the $\bar{X}$ and R charts are produced using ODS Graphics. The LIMITS= option in the PROC SHEWHART statement specifies the data set containing the control limits. By default, this information is read from the first observation in the LIMITS= data set for which

  • the value of _VAR_ matches the process name Diameter

  • the value of _SUBGRP_ matches the subgroup-variable name Batch

The charts are shown in Figure 18.113.

Figure 18.113: $\bar{X}$ and R Charts for Second Set of Wafer Data (ODS Graphics)

X and Charts for Second Set of Wafer Data (ODS Graphics)


Note that the mean diameter of the 29th batch lies below the lower control limit in the $\bar{X}$ chart, signaling a special cause of variation.

In this example, the LIMITS= data set was created in a previous run of the SHEWHART procedure. You can also create a LIMITS= data set with the DATA step. See LIMITS= Data Set for details concerning the variables that you must provide.