Example 15.31 Applying Tests for Special Causes

[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.

Output 15.31.1 Tests for Special Causes Displayed on Chart
Tests for Special Causes Displayed on u Chart

Output 15.31.2 Tabular Form of Chart
u Chart for Fabric Defects
Tests=1 to 4

The SHEWHART Procedure

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