Western Electric:The Zone Rules

Western Electric:The Zone Rules

1. designed to detect process instability

   • Zone Rule No 1
     • Any single data points fall outside the 3 sigma limit from the certeline, any point that falls outside ZoneA beyond either the upper or lower control limit
   • Zone Rule No 2
     • 2 or 3 consecutive points fall beyond the 2 sigmas limit(in zone A or beyond) on the same side of the centerline
   • Zone Rule No 3
     • Four out of five consecutive points fall beyond the 1 sigma limit (in zone B or beyond) on the same side of the centerline
   • Zone Rule No 4
     • Seven or more consecutive points fall on the same side of the centerline in zone C or beyond