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The SHEWHART Procedure

Example 13.34 Applying Tests for Special Causes

[See SHWTEST 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.

The following statements create an chart for the gap width measurements in the data set Parts in Creating Charts for Means from Subgroup Summary Data and tabulate the results:

ods graphics on;
title 'Tests for Special Causes Applied to Gap Width Data';
proc shewhart history=Parts;
   xchart Partgap*Sample/ tests    = 1 to 5
                          odstitle = title
                          tabletests
                          nolegend
                          tablecentral
                          tablelegend
                          zonelabels;
run;
ods graphics off;

The chart is shown in Output 13.34.1 and the printed output is shown in Output 13.34.2. The TESTS= requests Tests 1, 2, 3, 4, and 5, which are described in Tests for Special Causes The TABLECENTRAL option requests a table of the subgroup means, control limits, and central line. The TABLETESTS option adds a column indicating which subgroups tested positive for special causes, and the TABLELEGEND option adds a legend describing the tests that were signaled.

The ZONELABELS option displays zone lines and zone labels on the chart. The zones are used to define the tests. The NOLEGEND option suppresses the subgroup sample size legend that is displayed by default in the lower left corner of the chart.

Output 13.34.1 and Output 13.34.2 indicate that Test 5 was positive at sample 14, signaling a possible shift in the mean of the process.

Output 13.34.1 Tests for Special Causes Displayed on an Chart
Tests for Special Causes Displayed on an X Chart

Output 13.34.2 Tabular Form of Chart
Tests for Special Causes Applied to Gap Width Data

The SHEWHART Procedure

Means Chart Summary for Partgap
Sample Subgroup
Sample
Size
3 Sigma Limits with n=5 for Mean Special
Tests
Signaled
Lower
Limit
Subgroup
Mean
Average
Mean
Upper
Limit
1 5 242.08741 270.00000 259.66667 277.24592  
2 5 242.08741 258.00000 259.66667 277.24592  
3 5 242.08741 248.00000 259.66667 277.24592  
4 5 242.08741 260.00000 259.66667 277.24592  
5 5 242.08741 273.00000 259.66667 277.24592  
6 5 242.08741 260.00000 259.66667 277.24592  
7 5 242.08741 259.00000 259.66667 277.24592  
8 5 242.08741 248.00000 259.66667 277.24592  
9 5 242.08741 260.00000 259.66667 277.24592  
10 5 242.08741 255.00000 259.66667 277.24592  
11 5 242.08741 268.00000 259.66667 277.24592  
12 5 242.08741 253.00000 259.66667 277.24592  
13 5 242.08741 273.00000 259.66667 277.24592  
14 5 242.08741 275.00000 259.66667 277.24592 5
15 5 242.08741 257.00000 259.66667 277.24592  
16 5 242.08741 269.00000 259.66667 277.24592  
17 5 242.08741 249.00000 259.66667 277.24592  
18 5 242.08741 264.00000 259.66667 277.24592  
19 5 242.08741 258.00000 259.66667 277.24592  
20 5 242.08741 248.00000 259.66667 277.24592  
21 5 242.08741 248.00000 259.66667 277.24592  

Test Descriptions
Test 5 Two out of three points in a row in Zone A or beyond

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