See SHWNP2 in the SAS/QC Sample LibraryThis 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 the section
Creating np 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 on; title1'np Chart for the Number of Failing Circuits'; title2 'Tests=1 to 4'; proc shewhart data=Circuit3; npchart Fail*Batch / subgroupn = 500 tests=1 to 4 table tabletest tablelegend zones zonelabels odstitle = title odstitle2 = title2; run;
The chart is shown in Output 17.18.1, and the printed output is shown in Output 17.18.2. 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 counts 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. The ZONELABELS option displays zone lines and zone labels on the chart. The zones are used to define the tests.
Output 17.18.1 and Output 17.18.2 indicate that Test 1 is positive at batch 2 and Test 3 is positive at batch 10.
Output 17.18.1: Tests for Special Causes Displayed on Chart
Output 17.18.2: Tabular Form of Chart
np Chart for the Number of Failing Circuits |
Tests=1 to 4 |
np Chart Summary for Fail | |||||
---|---|---|---|---|---|
Batch | Subgroup Sample Size |
3 Sigma Limits with n=500 for Number | Special Tests Signaled |
||
Lower Limit |
Subgroup Number |
Upper Limit |
|||
1 | 500 | 0.60851449 | 12.000000 | 19.391486 | |
2 | 500 | 0.60851449 | 21.000000 | 19.391486 | 1 |
3 | 500 | 0.60851449 | 16.000000 | 19.391486 | |
4 | 500 | 0.60851449 | 9.000000 | 19.391486 | |
5 | 500 | 0.60851449 | 3.000000 | 19.391486 | |
6 | 500 | 0.60851449 | 4.000000 | 19.391486 | |
7 | 500 | 0.60851449 | 6.000000 | 19.391486 | |
8 | 500 | 0.60851449 | 9.000000 | 19.391486 | |
9 | 500 | 0.60851449 | 11.000000 | 19.391486 | |
10 | 500 | 0.60851449 | 13.000000 | 19.391486 | 3 |
11 | 500 | 0.60851449 | 12.000000 | 19.391486 | |
12 | 500 | 0.60851449 | 7.000000 | 19.391486 | |
13 | 500 | 0.60851449 | 2.000000 | 19.391486 | |
14 | 500 | 0.60851449 | 14.000000 | 19.391486 | |
15 | 500 | 0.60851449 | 9.000000 | 19.391486 | |
16 | 500 | 0.60851449 | 8.000000 | 19.391486 | |
17 | 500 | 0.60851449 | 14.000000 | 19.391486 | |
18 | 500 | 0.60851449 | 10.000000 | 19.391486 | |
19 | 500 | 0.60851449 | 11.000000 | 19.391486 | |
20 | 500 | 0.60851449 | 9.000000 | 19.391486 |
Test Descriptions | |
---|---|
Test 1 | One point beyond Zone A (outside control limits) |
Test 3 | Six points in a row steadily increasing or decreasing |