How to Interpret a Control Chart?
Remark:It is based on the Normal Distribution.
SPC Trend Rules
Rule#1: A single point beyond either control limit
Uses: Detects very large/sudden shifts
False alarm rate: 0.27%
Rule# 2: 9 consecutive points on the same side of the centerline
Uses: Detects small shifts or trends
False alarm rate: 0.39%
Rule# 3: 6 consecutive points steadily increasing or Decreasing
Uses: Detects strong trends
Rule# 4: 14 (or more) consecutive points are alternating up and down.
Uses: Detects systematic effects, such as alternating machines, operators, suppliers, etc.
Rule# 5: 2 out of 3 consecutive points at least 2 std dev beyond the centerline, on the same side
Uses: Detects large changes
False alarm rate: 0.30%
Rule# 6: 4 out of 5 consecutive points on the chart are more than 1 std dev away from the CL
Uses: Detects moderate-sized changes
False alarm rate: 0.53%
Rule# 7: 15 (or more) consecutive points are within 1 std dev of the CL
Uses: Detects a decrease in process variation
Rule# 8: 8 (or more) consecutive points are on both sides of the CL, but none are within 1 std dev of it.
Uses: Detects an increase in process variation