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