Example 42.1: Applying Tests for Special Causes
See SHWTEST in the SAS/QC Sample Library
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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
at "Creating Charts for Means from Subgroup Summary Data"
and tabulate the results:
title 'Tests for Special Causes Applied to Gap Width Data';
symbol v=dot c=yellow;
proc shewhart history=parts;
xchart partgap*sample/ tests = 1 to 5
ltests = 20
tabletests
tablecentral
tablelegend
zonelabels
nolegend
cframe = vigb
cinfill = vlib
ctests = red
cconnect = yellow;
run;
The
chart is shown in Output 42.1.1 and the
printed output is shown in Output 42.1.2.
The TESTS= requests Tests 1, 2, 3, 4, and 5, which are described
in Chapter 48, "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 LTESTS= option
specifies the line type used to connect the points in test patterns
that were signaled.
The NOLEGEND option suppresses
the subgroup sample size legend that is displayed by default in the
lower left corner of the chart.
Output 42.1.1 and Output 42.1.2 indicate that
Test 5 was positive at sample 14,
signaling a possible shift in the mean of the process.
Output 42.1.1: Tests for Special Causes Displayed on an
Chart
Output 42.1.2: Tabular Form of
Chart
Tests for Special Causes Applied to Gap Width Data |
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 |
|
Copyright © 1999 by SAS Institute Inc., Cary, NC, USA. All rights reserved.