Other Unnatural Patterns
The Handbook also identifies patterns that require consideration of both the upper and lower halves of the control chart together for identification:
| Pattern | Description | Rule |
|---|---|---|
| Stratification | Variation small relative to control limits | Fifteen consecutive points fall within ±1σ (zone C) |
| Mixture | The tendency to avoid the centerline | Eight consecutive points on both sides of the centerline with no points falling in zone C. |
| Systematic | Negative autocorrelation—a long series of observations that alternate high-low-high-low | (No rule given) |
| Repetition | The tendency of one chart to follow the same pattern as its predecessor | (No rule given) |
| Trend | Sustained drift or shift in the long-term mean | A series of out-of-control points in the lower zones followed by a series of out-of-control points in the upper zones or vice versa A series of points without a change in direction |
Read more about this topic: Western Electric Rules
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