Western Electric Rules

There are four Western Electric rules for control charts

Western Electric RulesĀ are the original set of four control chart rules used to conduct stability analysis. Western Electric RulesĀ can be used with all control charts and are sometimes referred to as WECO Rules.

What are the four Western Electric control chart rules?

  1. One point above UCL or below LCL
  2. Two points above/below 2 sigma
  3. Four out of five points above/below 1 sigma
  4. 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

weco rule 1

Western Electric Rule 2:
Two points above/below 2 sigma

weco rule 2

Western Electric Rule 3
Four out of five points above/below 1 sigma

WECO rule 3

Western Electric Rule 4
Eight points in a row above/below the center line

weco rule 4


Western Electric rules and many other rule sets are available in QI Macros add-in for Excel

western electric control chart rules

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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