Zone Rules
The most important of the Western Electric rules are the zone rules, designed to detect process instability—and the presence of assignable causes. There are four basic rules that deal with appraising runs of observations within the various zones:
| Rule | Description | Chart Example |
|---|---|---|
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Any single data point falls outside the 3σ limit from the centerline (i.e., any point that falls outside Zone A, beyond either the upper or lower control limit) | |
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Two out of three consecutive points fall beyond the 2σ limit (in zone A or beyond), on the same side of the centerline | |
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Four out of five consecutive points fall beyond the 1σ limit (in zone B or beyond), on the same side of the centerline | |
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Eight consecutive points fall on the same side of the centerline (in zone C or beyond) |
These rules are evaluated for one side of the center line (one half of the control band) at a time (e.g., first the centerline to the upper control limit, then the centerline to the lower control limit).
Data satisfying any of these conditions as indicated by the control chart provide the justification for investigating the process to discover whether assignable causes are present and can be removed. Note that there is always a possibility of false positives: Assuming observations are normally distributed, one expects Rule 1 to be triggered by chance one out of every 370 observations on average. The false alarm rate rises to one out of every 91.75 observations when evaluating all four rules.
Read more about this topic: Western Electric Rules
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