Editing Control chart (section)

==Rules for detecting signals==

The most common sets are:

*The [[Western Electric rules]]
*The [[Donald J. Wheeler|Wheeler]] rules (equivalent to the Western Electric zone tests<ref>{{Cite book
 | last1 = Wheeler
 | first1 = Donald J.
 | last2 = Chambers
 | first2 = David S.
 | author-link = Donald J. Wheeler
 | date = 1992
 | title = Understanding statistical process control
 | edition = 2
 | location = [[Knoxville, Tennessee]]
 | publisher = SPC Press
 | isbn = 978-0-945320-13-5
 | oclc = 27187772
 | page = 96
 | title-link = statistical process control
 }}</ref>)
*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 hypotheses suggested by the data|testing effects suggested by the data]].