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Project Management & Agile · Advanced

Western Electric Rules

Set of decision rules for detecting non-random patterns on control charts.

Definition

Examples: one point beyond 3 sigma; two of three consecutive points beyond 2 sigma on the same side; four of five beyond 1 sigma; eight consecutive points on one side of the centerline. Nelson rules extend these to eight tests.

Example

Two of three points beyond 2 sigma trigger an investigation.

Exam tip

PMI simplifies with the rule of seven; ASQ uses the formal rules.

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