Nelson Rules for Control Charts
Nelson rules include the four Western Electric rules plus four more
Nelson Rules were developed in the 1950s and can be used with any control chart.
What are the Nelson control chart rules?
- One point above UCL or below LCL
- Two points above/below 2 sigma
- Three out of four points above/below 1 sigma
- Eight points in a row above/below the center line
- Six points in a row ascending or descending (trend)
- 15 points in a row "hugging" the center line (between -1 and +1 sigma)
- 14 points in a row alternating up and down
- Eight points in a row above 1 sigma or below -1 sigma
The first four rules are the same as the Western Electric rules. You can view examples of the Western Electric rules here.
Here are examples of the Nelson rules 5, 6, 7, and 8
Nelson Rule 5
Six points in a row ascending or descending
Nelson Rule 6
15 points in a row "hugging" the center line
Nelson Rule 7
14 points in a row alternating up and down
Nelson Rule 8
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 than 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.
Nelson Rules and many others are available to create control charts in QI Macros add-in
QI Macros use Montgomery's Rules as the default. However, you can easily switch to Nelson rules using the QI Macros menu. See How to Change Control Chart Rules in QI Macros
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