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 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.
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