西方电气规则和尼尔森规则

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The 4 common Western Electric tests of instability are:

• Point outside the control limits.
• 2 of 3 points between 2s & 3s from the mean.
• 4 of 5 points between 1s & 3s from the mean.
• 8 points in a row on one side of the centerline.
  
  

The Nelson Rules were developed in the 1980s by Dr. Lloyd Nelson. Dr. Nelson put numbers to some of the Western Electric Other Unnatural Patterns. Nelson Rules are:

• 1 point outside of control limits.
• 2 of 3 points in zone A (between 2s and 3s from the mean).
• 4 of 5 in zones A or B (between 1s and 3s from the mean).
• Run of 9 on one side of the mean.
• 15 in a row near the centerline.
• Trend of 6 points in a row increasing or decreasing.
• 8 in a row not within 1s of the mean (on both sides of the mean).
• 14 points in a row that alternate up and down.
  
  

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