The Nelson Rules are eight tests for special causes in control charts, published by Lloyd S. Nelson in 1984 in the Journal of Quality Technology as an extension of the Western Electric Rules. Each rule flags a non-random pattern — such as one point beyond 3σ or nine points on one side of the mean — to separate assignable from inherent variation. More active rules mean higher sensitivity but more false alarms.

Nelson Rules

Details
Full Name

Nelson Rules (Tests for Special Causes)

Core Concepts:

  • 8 rules for detecting non-random patterns in Control Charts

    Rule 1

    One point beyond 3σ (Outlier)

    Rule 2

    9 consecutive points on the same side of the mean (Shift/Bias)

    Rule 3

    6 consecutive points steadily increasing or decreasing (Trend)

    Rule 4

    14 points alternating up and down (Oscillation)

    Rule 5

    2 out of 3 points beyond 2σ on the same side

    Rule 6

    4 out of 5 points beyond 1σ on the same side

    Rule 7

    15 points within 1σ (suspiciously low variance)

    Rule 8

    8 points outside ±1σ, but none beyond ±3σ (systematic oscillation)

    Common Cause vs. Special Cause

    Distinguishing inherent from assignable variation

    Zones A/B/C

    Dividing the Control Chart into 6 zones (each 1σ wide)

    False Positive Trade-off

    More active rules = higher sensitivity, but more false alarms

    Key Proponent

    Lloyd S. Nelson (1984, Journal of Quality Technology)

Relationship to Other Anchors:

Control Chart (Shewhart)

Nelson Rules are applied to Control Charts

SPC

Nelson Rules are a tool within Statistical Process Control

Western Electric Rules

Predecessor; Nelson Rules extend these with Rules 5-8

When to Use:

  • Detecting non-random patterns in time series data

  • Process monitoring in manufacturing, pharmaceuticals, healthcare

  • Potential application in IT monitoring (memory leaks, performance degradation)

  • Quality assurance in Six Sigma / DMAIC Control Phase