Example 13.22 Applying Tests for Special Causes

[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
;
run;

The following statements create the chart, apply several tests to the chart, and tabulate the results:

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 13.22.1, and the printed output is shown in Output 13.22.2. The TESTS= option requests Tests 1, 2, 3, and 4, which are described in Tests for Special Causes 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 13.22.1 and Output 13.22.2 indicate that Test 1 is positive at batch 2 and Test 3 is positive at batch 10.

Output 13.22.1 Tests for Special Causes Displayed on Chart

Output 13.22.2 Tabular Form of Chart

p Chart for the Proportion of Failing Circuits

Tests = 1 to 4

The SHEWHART Procedure