What are the Nelson Rules for Control Charts?
QI Macros makes it easy to select and apply Nelson rules to control charts.
Draw control charts with QI Macros
- Select your data.
- Click on QI Macros menu > Control Charts (SPC) > XmR
- QI Macros will do the math and analyze the chart for you.
List of the eight Nelson Rules for interpreting control charts
Nelson Rules were developed in the 1950s and can be used with any control chart.
They include four Western Electric rules (1-4) plus four more (5-8).
- One point above UCL or below LCL
- Two points above/below 2 sigma
*Nelson Rules state that 2 out of 3 points above/below 2 sigma MUST be above OR below the centerline*
- Four out of five 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
View examples of Western Electric rules 1-4
Examples of 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.
List of 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 Nelson rules using the QI Macros menu.
- How to Change Control Chart Rules in QI Macros.
- Stability Analysis Using Control Chart Rules
- Westgard Rules
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