选自：https://en.wikipedia.org/wiki/Western_Electric_rules
Asymmetric control limits[edit]
The zone rules presented above apply to control charts with symmetric control limits. The handbook provides additional guidelines for control charts where the control limits are not symmetrical, as for R charts and p-charts. For {\displaystyle {\bar {x}}} and R charts (which plot the behavior of the subgroup range), the Handbook recommends using the zone rules above for subgroups of sufficient size (five or more). For small sample subgroups, the Handbook recommends: Small-sample R chart rules | Any single data point falling above the +3σ limit | Two consecutive points falling above the +2σ limit (in the upper zone A or above) | Three consecutive points falling above the +1σ limit (in the upper zone B or above) | Seven consecutive points falling above the centerline (in the upper zone C or above) | Ten consecutive points falling below the centerline (in the lower zone C or below) | Six consecutive points falling below the -1σ limit (in the lower zone B or below) | Four consecutive points falling below the -2σ limit (in the lower zone A) |
For other control charts based on skewed distributions, the Handbook recommends:
- Finding the probabilities associated with each zone
- Devising tests that should result in the desired sensitivity to the presence of assignable causes
The above eight rules apply to a chart of a variable value. A second chart, the moving range chart, can also be used but only with rules 1, 2, 3 and 4.
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发表于 2016-10-12 20:46:49
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