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
Famous quotes containing the words unnatural and/or patterns:
“Women who assume authority are unnatural. Unnatural women are lesbians. Therefore all the leaders of the women’s movement were presumed to be lesbians.”
—Jane O’Reilly, U.S. feminist and humorist. The Girl I Left Behind, ch. 8 (1980)
“One can describe a landscape in many different words and sentences, but one would not normally cut up a picture of a landscape and rearrange it in different patterns in order to describe it in different ways. Because a photograph is not composed of discrete units strung out in a linear row of meaningful pieces, we do not understand it by looking at one element after another in a set sequence. The photograph is understood in one act of seeing; it is perceived in a gestalt.”
—Joshua Meyrowitz, U.S. educator, media critic. “The Blurring of Public and Private Behaviors,” No Sense of Place: The Impact of Electronic Media on Social Behavior, Oxford University Press (1985)