Tag Archive | SPC Trend Rules

SPC Trend Rules

How to Interpret a Control Chart?

control chart

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%
Example:

SPC Trend Rule#1

SPC Trend Rule#1

Rule# 2: 9 consecutive points on the same side of the centerline
Uses: Detects small shifts or trends
False alarm rate: 0.39%
Example:

SPC Trend Rule#2

SPC Trend Rule#2

Rule# 3: 6 consecutive points steadily increasing or Decreasing
Uses: Detects strong trends
Example:

SPC Trend Rule#3

SPC Trend Rule#3

Rule# 4: 14 (or more) consecutive points are alternating up and down.
Uses: Detects systematic effects, such as alternating machines, operators, suppliers, etc.
Example:

SPC Trend Rule#4

SPC Trend Rule#4

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%
Example:

SPC Trend Rule#5

SPC Trend Rule#5

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%
Example:

SPC Trend Rule#6

SPC Trend Rule#6

Rule# 7: 15 (or more) consecutive points are within 1 std dev of the CL
Uses: Detects a decrease in process variation
Example:

SPC Trend Rule#7

SPC Trend Rule#7

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
Example:

SPC Trend Rule#8

SPC Trend Rule#8