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

Output 13.34.2 Tabular Form of Chart

Tests for Special Causes Applied to Gap Width Data

The SHEWHART Procedure