Western Electric Rules
There are four Western Electric rules for control charts
Western Electric Rules are the original set of four control chart rules used to conduct stability analysis. Western Electric Rules can be used with all control charts and are sometimes referred to as WECO Rules.
What are the four Western Electric control chart rules?
- One point above UCL or below LCL
- Two points above/below 2 sigma
- Four out of five points above/below 1 sigma
- Eight points in a row above/below the center line
These were expanded by Nelsons Rules in the 1950s.
Here are examples of Western Electric Rules 1, 2, 3 and 4
Western Electric Rule 1:
One point above UCL or below LCL
Western Electric Rule 2:
Two points above/below 2 sigma
Western Electric Rule 3
Four out of five points above/below 1 sigma
Western Electric Rule 4
Eight points in a row above/below the center line
Western Electric rules and many other rule sets are available in QI Macros add-in for Excel
QI Macros use Montgomery's Rules as the default. However, you can easily switch to the Western Electric Rules using the QI Macros menu. See How to Change Control Chart Rules in QI Macros
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