Rules For Detecting Signals
The most common sets are:
- The Western Electric rules
- The Wheeler rules (equivalent to the Western Electric zone tests)
- The Nelson rules
There has been particular controversy as to how long a run of observations, all on the same side of the centre line, should count as a signal, with 6, 7, 8 and 9 all being advocated by various writers.
The most important principle for choosing a set of rules is that the choice be made before the data is inspected. Choosing rules once the data have been seen tends to increase the Type I error rate owing to testing effects suggested by the data.
Read more about this topic: Control Chart
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