[See SHWUEX1 in the SAS/QC Sample Library]This example illustrates how you can apply tests for special causes to make charts more sensitive to special causes of variation.
A textile company inspects rolls of fabric for defects. The rolls are one meter wide and 30 meters long. The following statements create a SAS data set named Fabric3, which contains the number of fabric defects for 20 rolls of fabric:
data Fabric3; input Roll Defects @@; datalines; 1 6 2 9 3 14 4 17 5 3 6 8 7 9 8 2 9 14 10 1 11 3 12 5 13 6 14 9 15 10 16 12 17 11 18 4 19 9 20 4 ;
The following statements create a chart and tabulate the information on the chart. The chart and tables are shown in Output 15.31.1 and Output 15.31.2.
ods graphics on; title1 'u Chart for Fabric Defects'; title2 'Tests=1 to 4'; proc shewhart data=Fabric3; uchart Defects*Roll / subgroupn = 30 tests = 1 to 4 odstitle = title odstitle2 = title2 tabletests zonelabels; run;
The TESTS= option requests Tests 1, 2, 3, and 4, which are described in Tests for Special Causes: SHEWHART Procedure. Only Tests 1, 2, 3, and 4 are recommended for charts. The ZONELABELS option requests the zone lines, which are used to define the tests, and displays labels for the zones. The TABLETESTS option requests a table of the values of and the control limits, together with a column indicating the subgroups at which the tests are positive.
Output 15.31.1 and Output 15.31.2 indicate that Test 1 is positive for Roll 4 and Test 3 is positive at Roll 15.
u Chart for Fabric Defects |
Tests=1 to 4 |
u Chart Summary for Defects | |||||
---|---|---|---|---|---|
Roll | Subgroup Sample Size |
3 Sigma Limits with n=30 for Count per Unit |
Special Tests Signaled |
||
Lower Limit |
Subgroup Count per Unit |
Upper Limit |
|||
1 | 30.0000 | 0 | 0.20000000 | 0.53928480 | |
2 | 30.0000 | 0 | 0.30000000 | 0.53928480 | |
3 | 30.0000 | 0 | 0.46666667 | 0.53928480 | |
4 | 30.0000 | 0 | 0.56666667 | 0.53928480 | 1 |
5 | 30.0000 | 0 | 0.10000000 | 0.53928480 | |
6 | 30.0000 | 0 | 0.26666667 | 0.53928480 | |
7 | 30.0000 | 0 | 0.30000000 | 0.53928480 | |
8 | 30.0000 | 0 | 0.06666667 | 0.53928480 | |
9 | 30.0000 | 0 | 0.46666667 | 0.53928480 | |
10 | 30.0000 | 0 | 0.03333333 | 0.53928480 | |
11 | 30.0000 | 0 | 0.10000000 | 0.53928480 | |
12 | 30.0000 | 0 | 0.16666667 | 0.53928480 | |
13 | 30.0000 | 0 | 0.20000000 | 0.53928480 | |
14 | 30.0000 | 0 | 0.30000000 | 0.53928480 | |
15 | 30.0000 | 0 | 0.33333333 | 0.53928480 | 3 |
16 | 30.0000 | 0 | 0.40000000 | 0.53928480 | |
17 | 30.0000 | 0 | 0.36666667 | 0.53928480 | |
18 | 30.0000 | 0 | 0.13333333 | 0.53928480 | |
19 | 30.0000 | 0 | 0.30000000 | 0.53928480 | |
20 | 30.0000 | 0 | 0.13333333 | 0.53928480 |