Quality and Reliability Methods - SAS

42 Statistical Control Charts Chapter 2
Tests for Special Causes
Figure 2.18 Illustration of Special Causes Tests 1
Test 1: One point beyond Zone A
A
B
C
C
B
A
1
UCL
Avg
LCL
Test 3: Six points in a row steadily increasing
or decreasing
A
B
C
C
B
A
3
UCL
Avg
LCL
Test 2: Nine points in a row in a single (upper
or lower) side of Zone C or beyond
A
UCL
B
C
C
B
A
2
Avg
LCL
Test 4: Fourteen points in a row alternating
up and down
A
UCL
B
C
C
B
A
4 Avg
LCL
Test 5: Two out of three points in a row in Test 6: Four out of five points in a row in
Zone A or beyond
Zone B or beyond
A 5
UCL
5 A
6
UCL
B
B
C
Avg C
C
C
Avg
B
A
5
LCL
B
A
LCL
Test 7: Fifteen points in a row in Zone C
(above and below the center line)
A
UCL
B
C
7 Avg
C
B
A
LCL
Test 8: Eight points in a row on both sides of
the center line with none in Zone C
A
UCL
B
C
Avg
C
B
8
A
LCL
Westgard Rules
Westgard rules are implemented under the Westgard Rules submenu of the Control Chart platform. The
different tests are abbreviated with the decision rule for the particular test. For example, 1 2s refers to a test
where one point is two standard deviations away from the mean.
1. Nelson (1984, 1985)