[See SHWPEX1 in the SAS/QC Sample Library]This example shows how you can apply tests for special causes to make charts more sensitive to special causes of variation. The following statements create a SAS data set named Circuit3, which contains the number of failing circuits for 20 batches from the circuit manufacturing process introduced in Creating p Charts from Count Data:
data Circuit3; input Batch Fail @@; datalines; 1 12 2 21 3 16 4 9 5 3 6 4 7 6 8 9 9 11 10 13 11 12 12 7 13 2 14 14 15 9 16 8 17 14 18 10 19 11 20 9 ;
The following statements create the chart, apply several tests to the chart, and tabulate the results:
ods graphics off; title1'p Chart for the Proportion of Failing Circuits'; title2 'Tests = 1 to 4'; proc shewhart data=Circuit3; pchart Fail*Batch / subgroupn = 500 tests = 1 to 4 zones zonelabels ltests = 20 table tabletest tablelegend; run;
The chart is shown in Output 15.22.1, and the printed output is shown in Output 15.22.2. The TESTS= option requests Tests 1, 2, 3, and 4, which are described in Tests for Special Causes: SHEWHART Procedure. The TABLETESTS option requests a table of proportions of nonconforming items and control limits, with a column indicating which subgroups tested positive for special causes. The TABLELEGEND option adds a legend describing the tests that are positive.
The ZONELABELS option displays zone lines and zone labels on the chart. The zones are used to define the tests. The LTESTS= option specifies the line type used to connect the points in a pattern for a test that is signaled.
Output 15.22.1 and Output 15.22.2 indicate that Test 1 is positive at batch 2 and Test 3 is positive at batch 10.
p Chart for the Proportion of Failing Circuits |
Tests = 1 to 4 |
p Chart Summary for Fail | |||||
---|---|---|---|---|---|
Batch | Subgroup Sample Size |
3 Sigma Limits with n=500 for Proportion | Special Tests Signaled |
||
Lower Limit |
Subgroup Proportion |
Upper Limit |
|||
1 | 500 | 0.00121703 | 0.02400000 | 0.03878297 | |
2 | 500 | 0.00121703 | 0.04200000 | 0.03878297 | 1 |
3 | 500 | 0.00121703 | 0.03200000 | 0.03878297 | |
4 | 500 | 0.00121703 | 0.01800000 | 0.03878297 | |
5 | 500 | 0.00121703 | 0.00600000 | 0.03878297 | |
6 | 500 | 0.00121703 | 0.00800000 | 0.03878297 | |
7 | 500 | 0.00121703 | 0.01200000 | 0.03878297 | |
8 | 500 | 0.00121703 | 0.01800000 | 0.03878297 | |
9 | 500 | 0.00121703 | 0.02200000 | 0.03878297 | |
10 | 500 | 0.00121703 | 0.02600000 | 0.03878297 | 3 |
11 | 500 | 0.00121703 | 0.02400000 | 0.03878297 | |
12 | 500 | 0.00121703 | 0.01400000 | 0.03878297 | |
13 | 500 | 0.00121703 | 0.00400000 | 0.03878297 | |
14 | 500 | 0.00121703 | 0.02800000 | 0.03878297 | |
15 | 500 | 0.00121703 | 0.01800000 | 0.03878297 | |
16 | 500 | 0.00121703 | 0.01600000 | 0.03878297 | |
17 | 500 | 0.00121703 | 0.02800000 | 0.03878297 | |
18 | 500 | 0.00121703 | 0.02000000 | 0.03878297 | |
19 | 500 | 0.00121703 | 0.02200000 | 0.03878297 | |
20 | 500 | 0.00121703 | 0.01800000 | 0.03878297 |
Test Descriptions | |
---|---|
Test 1 | One point beyond Zone A (outside control limits) |
Test 3 | Six points in a row steadily increasing or decreasing |