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What are Western Electric Rules?

What are Western Electric Rules?

The Western Electric rules (WECO Rules) are decision rules in statistical process control for detecting out-of-control or non-random conditions on control chart. Locations of the observations relative to the control chart control limits (typically at ±3 standard deviations) and centerline indicate whether the process in question should be investigated for assignable causes. Their purpose is to ensure that line workers and engineers interpret control charts in a uniform way.

The Western Electric rules were codified by a specially appointed committee of the manufacturing division of the Western Electric Company and appeared in the first edition of a 1956 handbook, that became a standard text of the field. An updated version was introduced in 1984 by Lloyd S. Nelson, in order to make the probability of detecting an out-of-control condition by chance approximately equal across all tests. They were first published in the October 1984 issue of the Journal of Quality Technology.

This article will focus on the Nelson’s version of the WECO Rules.

What are the different types of WECO Rules and what do they indicate?

The rules are applied to a control chart on which the magnitude of some variable is plotted against time. The rules are based on the mean value and the standard deviation of the samples.



Chart Example

Problem Indicated

Rule 1

One point is more than 3 standard deviations from the mean.

One sample (two shown in this case) is grossly out of control.

Rule 2

Nine (or more) points in a row are on the same side of the mean.

Some prolonged "bias" exists

Rule 3

Six (or more) points in a row are continually increasing (or decreasing).

A "trend" exists

Rule 4

Fourteen (or more) points in a row alternate in direction, increasing then decreasing

This much "oscillation" is beyond noise.

Note that the rule is considered with directionality only. The position of the mean and the size of the standard deviation have no bearing.

Rule 5

Two (or three) out of the three points in a row are more than 2 standard deviations from the mean in the same direction.

There is a medium tendency for samples to be mediumly out of control.

The side of the mean for the third point is unspecified.

Rule 6

Four (or five) out of five points in a row are more than 1 standard deviation from the mean in the same direction.

There is strong tendency for samples to be slightly out of control.

The side of the mean for the fifth point is unspecified.

Rule 7

Fifteen points in a row are all within 1 standard deviation of the mean on either side of the mean.

With 1 standard deviation, greater variation would be expected.

Rule 8

Eight points in a row exist with none within 1 standard deviation of the mean and the points are in both directions of the mean.

Jumping from above to below while missing the first standard deviation band is rarely random

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Abeer Singhal

Abeer has over a decade of experience in semiconductor processing and advanced manufacturing. He is co-inventor on U.S. patents in the field of semiconductor manufacturing and has extensive experience implementing advanced manufacturing systems for high volume and complexity products. Prior to his role at Sentient, Abeer has worked as an equipment technician, process engineer, advanced process control systems engineer, and manufacturing systems architect. He has a degree in Microelectronic Engineering.