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370 Six SigMa DemystifieD
Methodology
The Western Electric Statistical Quality Control Handbook defined zones using
the sigma levels of the normal distribution, as shown in Figure F.40.
Run test 1 (Western Electric): One subgroup beyond 3σ, an
indication that the process mean has shifted. See Figure F.41.
Run test 2 (Nelson): Nine consecutive subgroups on the same side
of the average. (Note: Western Electric uses eight consecutive
points on the same side of the average). See Figure F.41.
Run test 3 (Nelson): Six consecutive subgroups increasing or
decreasing. See Figure F.42.
Run test 4 (Nelson): Fourteen consecutive subgroups alternating up
and down. See Figure F.42.
Run test 5 (Western Electric): Two of three consecutive subgroups
beyond 2σ. See Figure F.43.
Run test 6 (Western Electric): Four of five consecutive subgroups
beyond 1σ. See Figure F.43.
Run test 7 (Western Electric): Fifteen consecutive subgroups
between +1σ sigma and –1σ. See Figure F.44.
Figure F.40 Normal distribution zones.
