IRCHART Statement: SHEWHART Procedure

Example 17.11 Applying Tests for Special Causes

Note: See IRCHART with Tests for Special Causes in the SAS/QC Sample Library.

This example illustrates how you can apply tests for special causes to make an individual measurements chart more sensitive to special causes of variation. The following statements create a data set named Engines, which contains the weights for 25 jet engines: 

data Engines;
   input ID Weight @@;
   label Weight='Engine Weight (lbs)'
         ID    ='Engine ID Number';
   datalines;
1711 1270   1712 1258   1713 1248   1714 1260
1715 1263   1716 1260   1717 1259   1718 1240
1719 1260   1720 1246   1721 1238   1722 1253
1723 1249   1724 1245   1725 1251   1726 1252
1727 1249   1728 1274   1729 1258   1730 1268
1731 1248   1732 1295   1733 1243   1734 1253
1735 1258
;

Individual measurements and moving range charts are used to monitor the weights. The following statements produce the tables shown in Output 17.11.1 and create the charts shown in Output 17.11.2: 

title 'Tests for Special Causes Applied to Jet Engine Weights';
ods graphics on;
proc shewhart data=Engines;
   irchart Weight*ID /
       tests    = 1 to 8
       test2run = 7
       odstitle = title
       tabletest
       zonelabels
       markers;
run;

The TESTS= option applies eight tests for special causes, which are described in Tests for Special Causes: SHEWHART Procedure. The TEST2RUN= option specifies the length of the pattern for Test 2. The TABLETESTS option requests a table of individual measurements, moving ranges, and control limits, and it adds a column indicating which measurements tested positive for special causes.

The ZONELABELS option displays zone lines and zone labels on the individual measurements chart. The zones are used to define the tests.

Output 17.11.1: Tabular Form of Individual Measurements and Moving Range Chart

Individual Measurements Chart Summary for Weight
ID 3 Sigma Limits with n=2 for Weight Special
Tests
Signaled 		3 Sigma Limits with n=2 for
Moving Range
Lower
Limit 		Weight Upper
Limit 		Lower
Limit 		Moving
Range 		Upper
Limit
1711 1220.4709 1270.0000 1291.3691   0 . 43.553759
1712 1220.4709 1258.0000 1291.3691   0 12.000000 43.553759
1713 1220.4709 1248.0000 1291.3691   0 10.000000 43.553759
1714 1220.4709 1260.0000 1291.3691   0 12.000000 43.553759
1715 1220.4709 1263.0000 1291.3691   0 3.000000 43.553759
1716 1220.4709 1260.0000 1291.3691   0 3.000000 43.553759
1717 1220.4709 1259.0000 1291.3691   0 1.000000 43.553759
1718 1220.4709 1240.0000 1291.3691   0 19.000000 43.553759
1719 1220.4709 1260.0000 1291.3691   0 20.000000 43.553759
1720 1220.4709 1246.0000 1291.3691   0 14.000000 43.553759
1721 1220.4709 1238.0000 1291.3691   0 8.000000 43.553759
1722 1220.4709 1253.0000 1291.3691   0 15.000000 43.553759
1723 1220.4709 1249.0000 1291.3691   0 4.000000 43.553759
1724 1220.4709 1245.0000 1291.3691   0 4.000000 43.553759
1725 1220.4709 1251.0000 1291.3691   0 6.000000 43.553759
1726 1220.4709 1252.0000 1291.3691 2 0 1.000000 43.553759
1727 1220.4709 1249.0000 1291.3691   0 3.000000 43.553759
1728 1220.4709 1274.0000 1291.3691   0 25.000000 43.553759
1729 1220.4709 1258.0000 1291.3691   0 16.000000 43.553759
1730 1220.4709 1268.0000 1291.3691   0 10.000000 43.553759
1731 1220.4709 1248.0000 1291.3691   0 20.000000 43.553759
1732 1220.4709 1295.0000 1291.3691 1 0 47.000000 43.553759
1733 1220.4709 1243.0000 1291.3691   0 52.000000 43.553759
1734 1220.4709 1253.0000 1291.3691   0 10.000000 43.553759
1735 1220.4709 1258.0000 1291.3691   0 5.000000 43.553759 




            

            


            

            
  

            


Output 17.11.2: Tests for Special Causes

               

                  
Tests for Special Causes

               

            

            


            

            
Output 17.11.1 and Output 17.11.2 indicate that Test 1 was positive for engine 1732 and Test 2 was positive for engine 1726. Test 1 detects one point beyond
               Zone A (outside the control limits) and Test 2 detects seven points (TEST2RUN=7) in a row on one side of the central line.