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

control chart rule 5

Nelson Rule 6
15 points in a row "hugging" the center line

control chart rule 6

Nelson Rule 7
14 points in a row alternating up and down

control chart rule 7

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

control chart rule 8

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.

Moving Range chart example with Nelson range rules