Western Electric Rules
The Western Electric Rules are decision rules for detecting "out-of-control" or non-random conditions on control charts.
The rules attempt to distinguish unnatural patterns from natural patterns based on several criteria:
- The absence of points near the centre line (identified as a mixture pattern)
- The absence of points near the control limits (identified as a stratification pattern)
- The presence of points outside the control limits (identified as an instability pattern)
- Other unnatural patterns (systematic (auto correlative), repetition, trend patterns)
To achieve this, the rules divide the chart into zones, measured in units of standard deviation (σ) between the centre line and control limits. You can apply these rules to a data set in WATS by checking the “Western Electric Rule” box.
WATS will highlight data points where one of the rules has been detected. Hold the mouse over for details.