The rules split the chart into zones at 1, 2 and 3 sigma and signal on any of four patterns: one point beyond 3 sigma; two of three consecutive points beyond 2 sigma on the same side; four of five beyond 1 sigma on the same side; or eight in a row on one side of the centerline. Every added rule makes the chart quicker to catch small shifts but raises the false alarm rate, from about 1 in 370 points for rule 1 alone to roughly 1 in 92 with all four.
Not past the limits yet, but it's two out of three in zone A, so by Western Electric rules it's a violation. Hold the lot.
Turn on every rule across 2,000 charts and someone gets paged for a false alarm every few minutes. Fabs usually run rule 1 everywhere and add the others only on key parameters.
Also heard as
- WECO rules
- run rules
- zone rules
Related terms
Control chart
Time-ordered plot of a process statistic with a centerline and limits that flag when the process has changed.
Run of seven
Seven consecutive chart points on one side of the centerline, or steadily rising or falling, read as a process shift.
Hugging the centerline
Chart pattern where points cluster unnaturally close to the centerline, usually a sign the limits are wrong.
OOC
Out of control: shorthand for a point or chart that has violated a control limit or run rule.
Special cause
Specific, identifiable source of variation that is not part of the process's normal behavior and shows up as a chart signal.