IRCHART Statement: SHEWHART Procedure

Example 15.11 Applying Tests for Special Causes

[See SHWIREX1 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 15.11.1 and create the charts shown in Output 15.11.2:

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 15.11.1 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
ID	Lower Limit	Weight	Upper Limit	Special Tests Signaled	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 15.11.2 Tests for Special Causes

Output 15.11.1 and Output 15.11.2 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.