Zone Control Chart: Automated Western Electric Run-Rule Judgment
What is the Zone Control Chart?
The zone chart divides the control chart into six zones — center ±1σ (C), ±2σ (B) and ±3σ (A) — and scores each point (C = 0, B = 1, A = 2); accumulated scores over consecutive points trigger rule violations. It is a graphical implementation of the Western Electric/Nelson rules — nine points on one side, six in a row increasing, and so on — unified into a single scoring system.
When to Use It
Use it when automatic, standardized rule testing on a conventional variables chart is required, when the eight run rules should be integrated into one computable score for software, and for sensitive monitoring of process patterns such as runs, trends and alternation. The zone chart is a common choice for quality information systems that need consistent, traceable judgments.
How to Use It (Step by Step)
Paste the data, and the tool colors the zones, accumulates the scores and flags the triggered rule with its number (for example one point beyond 3σ or nine consecutive points on one side). Read the AI interpretation of which rule fired and why, then investigate the special cause with a 4M1E approach.
Key Formulas / Example
C zone (within ±1σ) scores 0, B zone (±1–2σ) 1, A zone (±2–3σ) 2; for example, three consecutive points accumulating at least 4 points, or two of three points in zone A on the same side, trigger a signal. More rules mean more sensitivity and more false alarms, so select rules by process risk.