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Out of Control

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Definition

A term used in Statistical process control (SPC) to indicate a condition of process instability. A control chart signals an out-of-control condition in the process if certain rules are violated. The most widely used set of rules are from the Western Electric Handbook :

1. Any single point falls in the area beyond 3 Standard Deviations on either side of the center line.
2. Two out of three consecutive points fall in the area beyond 2 Standard Deviations on either side of the center line.
3. Four out of five consecutive points fall in the area beyond 1 standard deviation on either side of the center line.
4. Nine consecutive points fall on the same side of the center line.

Application

The Western Electric Rules.