Example 38.1: Applying Tests for Special Causes

PCHART Statement

Example 38.1: 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 p 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
   ;

The following statements create the p 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';
   symbol v=dot c=salmon;
   proc shewhart data=circuit3;
      pchart fail*batch / subgroupn = 500
                          tests     = 1 to 4
                          ltests    = 20
                          zonelabels
                          tabletest
                          tablelegend
                          cframe    = lib
                          cinfill   = bwh
                          coutfill  = yellow
                          czones    = lib
                          ctests    = cxfefefe
                          cconnect  = salmon;
   run;

The chart is shown in Output 38.1.1, and the printed output is shown in Output 38.1.2. The TESTS= option requests Tests 1, 2, 3, and 4, which are described in Chapter 48, "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 38.1.1 and Output 38.1.2 indicate that Test 1 is positive at batch 2 and Test 3 is positive at batch 10.

Output 38.1.1: Tests for Special Causes Displayed on p Chart

Output 38.1.2: Tabular Form of p Chart

p Chart for the Proportion of Failing Circuits

Tests = 1 to 4

The SHEWHART Procedure

p Chart Summary for fail
batch	Subgroup Sample Size	3 Sigma Limits with n=500 for Proportion			Special Tests Signaled
batch	Subgroup Sample Size	Lower Limit	Subgroup Proportion	Upper Limit	Special Tests Signaled
1	500	0.00121703	0.02400000	0.03878297
2	500	0.00121703	0.04200000	0.03878297	1
3	500	0.00121703	0.03200000	0.03878297
4	500	0.00121703	0.01800000	0.03878297
5	500	0.00121703	0.00600000	0.03878297
6	500	0.00121703	0.00800000	0.03878297
7	500	0.00121703	0.01200000	0.03878297
8	500	0.00121703	0.01800000	0.03878297
9	500	0.00121703	0.02200000	0.03878297
10	500	0.00121703	0.02600000	0.03878297	3
11	500	0.00121703	0.02400000	0.03878297
12	500	0.00121703	0.01400000	0.03878297
13	500	0.00121703	0.00400000	0.03878297
14	500	0.00121703	0.02800000	0.03878297
15	500	0.00121703	0.01800000	0.03878297
16	500	0.00121703	0.01600000	0.03878297
17	500	0.00121703	0.02800000	0.03878297
18	500	0.00121703	0.02000000	0.03878297
19	500	0.00121703	0.02200000	0.03878297
20	500	0.00121703	0.01800000	0.03878297

Test Descriptions
Test 1	One point beyond Zone A (outside control limits)
Test 3	Six points in a row steadily increasing or decreasing

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