What are the Western Electric Rules for Control Charts?

QI Macros knows the rules so that you don't have to!

Draw control charts with QI Macros

2. Click on QI Macros menu > Control Charts (SPC) > XmR
3. QI Macros will do the math and analyze the chart for you.

Four Western Electric Control Chart Rules.

Western Electric Rules are the original four rules used to interpret control charts. Western Electric Rules can be used with all control charts and are sometimes referred to as WECO Rules.

1. One point above UCL or below LCL
2. Two out of the three points either above or below 2 sigma
*WECO Rules state that 2 out of 3 points above/below 2 sigma MUST be above OR below the centerline. NOTE: Odd point can be anywhere.*
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.

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

Control chart rules 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.