Nelson Rules for Control Charts

Nelson Rules Expand on Western Electric Rules

Nelson Rules can be used with any control chart. They include the Western Electric set of control chart rules. These were expanded by Nelson in the 1950s.

Nelson Rules Used to Create Control Charts in QI Macros

Like traditional control chart rules, Nelson rules include Western Electric rules 1, 2, 3, and 4 and add four more rules:

1. One point above UCL or below LCL
2. Two points above/below 2 sigma
3. Three out of four points above/below 1 sigma
4. Eight points in a row above/below the center line
5. Six points in a row ascending or descending (trend)
6. 15 points in a row "hugging" the center line (between -1 and +1 sigma)
7. 14 points in a row alternating up and down
8. Eight points in a row above 1 sigma or below -1 sigma

Nelson Rule: 6 points in a row ascending or descending - trend

Nelson Rule: 15 points in a row "hugging" the center line

Nelson Rule: 14 points in a row alternating up and down

Nelson Rule: 8 points in a row above 1 sigma or below -1 sigma

Special Case: XmR Charts use Nelson Rule for Moving Range

"Any moving range (Ri) greater then 3.5*Rbar should be removed from the average moving range calculation."

In example below, point 20 (46.50) is greater than 3.5*11.14 (original Rbar), so it will be removed from Rbar Calculation which changes Rbar to 9.73 and UCL/LCL for both the X and R charts. 

Source: Control Charts for Individual Measurements, Lloyd S. Nelson, Journal of Quality Technology, Vol. 14, No. 34, July 1982.

Learn More...

• Stability Analysis Using Control Chart Rules
• Westgard Rules
• How to Change Control Chart Rules in QI Macros

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