Control chart rules are used to perform stability analysis
An unstable process is not predictable and is considered "out of control".
The concepts of process control and process stability are important because:
- a process must be stable before you can perform process capability analysis to determine if it meets customer specifications.
- a process must be stable before beginning an improvement project.
How does a control chart determine if a process is out of control?
- Control limits (± 1, 2, 3 sigma) are calculated from the data.
- Zones represent the space between the limits.
- Control chart rules are then applied to data points as they move through those zones.
- Unstable points and trends are identified for investigation.
QI Macros highlights unstable points and trends by turning them red.
QI Macros also turns the markers for unstable points into diamonds instead of squares.
This helps to identify unstable points and trends when printing control charts on a black and white printer.
Control chart rules used by various industries and experts
Control chart rules can vary slightly by industry and by statistician. However, most of the basic rules used to run stability analysis are the same.
QI Macros uses the Montgomery rules from Introduction to Statistical Process Control, 4th edition pp 172-175, Montgomery as its default. QI Macros also offers these other rules and makes it easy to change from one rule set to another using its Control Chart Rules sub-menu.
| # | Control Chart Rule |
West-gard | Nelson- Juran | AIAG | Mont-gomery | Western Electric | IHI |
|---|---|---|---|---|---|---|---|
| 1 | n points above UCL or below LCL |
1 | 1 | 1 | 1 | 1 | 1 |
| 2 | Zone A: n of n+1 points above/below 2 sigma |
2 | 2 | 2 | 2 | 2 | 2 |
| 3 | Zone B: n of n+1 points above/below 1 sigma |
4 | 4 | 4 | 4 | ||
| 4 | n points in a row above/ below center line |
8 | 9 | 7 | 8 | 8 | 8 |
| 5 | Trends of n points in a row increasing or decreasing |
7 | 6 | 6 | 6 | 6 | |
| 6 | Zone C: n points in a row inside Zone C (hugging) |
15 | 15 | 15 | 15 | ||
| 7 | n points in a row alternating up and down |
14 | 14 | 14 | |||
| 8 | Zone C:n points in a row outside Zone C |
8 | 8 | 8 | |||
| 9 | Zone B: 4 points above/ below 1 sigma; 2 points one above, one below 2 sigma |
4 |
The Evolution of Control Chart Rules
Western Electric Rules were first developed almost 100 years ago.
Nelson Rules expanded the set of rules to cover increasingly rare conditions.
- Westgard Rules are used with Levey Jennings Charts in laboratories. They are a slightly different subset of traditional rules with a couple of special rules.
- AIAG Rules are recommended by the Automotive Industry Action Group
- Healthcare Rules are recommended by the Institute for Healthcare Improvement
Learn More ...
- How to change the default control chart rules used by QI Macros
- How to create your own custom control chart rules
- Stability analysis options in QI Macros
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