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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.
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 ...
They are (1) a Stability Ratio which compares the long-term variability to the short-term variability, (2) an ANOVA Test which compares the within-subgroup variation to the between-subgroup variation, and (3) an Instability Ratio which compares the number of subgroups that have one or more violations of the Western Electric rules to the total ...
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.
Nelson rules are a method in process control of determining whether some measured variable is out of control (unpredictable versus consistent). Rules for detecting "out-of-control" or non-random conditions were first postulated by Walter A. Shewhart [ 1 ] in the 1920s.
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Western Electric Walter Andrew Shewhart (pronounced like "shoe-heart"; March 18, 1891 – March 11, 1967) was an American physicist, engineer and statistician. He is sometimes also known as the grandfather of statistical quality control and also related to the Shewhart cycle .