Motivation
The rules attempt to distinguish unnatural patterns from natural patterns based on several criteria:
- The absence of points near the centerline (identified as a mixture pattern)
- The absence of points near the control limits (identified as a stratification pattern)
- The presence of points outside the control limits (identified as an instability pattern)
- Other unnatural patterns (systematic (autocorrelative), repetition, trend patterns)
To achieve this, the rules divide the chart into zones, measured in units of standard deviation (σ) between the centerline and control limits, as follows:
| Zone | Region |
|---|---|
| Zone A | Within 2σ of the centerline and the control limit (3σ) |
| Zone B | Within 1σ and 2σ of the centerline |
| Zone C | Within 1σ of the centerline |
Zones A, B, and C are sometimes called the three sigma zone, two sigma zone, and one sigma zone, respectively.
Read more about this topic: Western Electric Rules
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—June Jordan (b. 1939)