Example 15.18 Applying Tests for Special Causes

[See SHWNP2 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 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 15.18.1, and the printed output is shown in Output 15.18.2. 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 15.18.1 and Output 15.18.2 indicate that Test 1 is positive at batch 2 and Test 3 is positive at batch 10.

Output 15.18.1 Tests for Special Causes Displayed on Chart
Tests for Special Causes Displayed on np Chart

Output 15.18.2 Tabular Form of Chart
np Chart for the Number of Failing Circuits
Tests=1 to 4

The SHEWHART Procedure

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