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The Western Electric rules are decision rules in statistical process control for detecting out-of-control or non-random conditions on control charts. [1] Locations of the observations relative to the control chart control limits (typically at ±3 standard deviations) and centerline indicate whether the process in question should be investigated for assignable causes.
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The Western Electric rules; The Wheeler rules (equivalent to the Western Electric zone tests [16]) 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.
They are a set of modified Western Electric rules, developed by James Westgard and provided in his books and seminars on quality control. [1] They are plotted on Levey–Jennings charts, wherein the X-axis shows each individual sample, and the Y-axis shows how much each one differs from the mean in terms of standard deviation (SD). The rules ...
The Nelson rules were first published in the October 1984 issue of the Journal of Quality Technology in an article by Lloyd S Nelson. [2] The rules are applied to a control chart on which the magnitude of some variable is plotted against time. The rules are based on the mean value and the standard deviation of the samples.
The Western Electric rules conclude that the process is out of control if: One point plots outside the 3σ-limits (the UCL and LCL). Two out of three consecutive points plot beyond a 2σ-limit. Four out of five consecutive points plot at a distance of 1σ or beyond from the centerline. Eight consecutive points plot on one side of the center line.
It seems like every web page I visit that mentions Western Electric rules, says that rule 4 is violated if just eight consecutive points fall on the same side of the centerline.--216.254.190.174 13:09, 3 July 2009 (UTC)
The electric power transmission grid of the contiguous United States consists of 120,000 miles (190,000 km) of lines operated by 500 companies. The Western Interconnection is a wide area synchronous grid and one of the two major alternating current (AC) power grids in the North American power transmission grid .