zone a b c

**Zone A B C** is the **three-band sigma partition of a control chart used to interpret point concentration and pattern risk** - the zones translate statistical distance from centerline into practical surveillance signals. **What Is Zone A B C?** - **Definition**: Partitioning each side of the centerline into C zone (0 to 1 sigma), B zone (1 to 2 sigma), and A zone (2 to 3 sigma). - **Probability Meaning**: Most common-cause points occur in Zone C, fewer in Zone B, and rare points in Zone A. - **Chart Utility**: Zone occupancy patterns are used by Western and Nelson-style detection rules. - **Interpretation Context**: Requires stable control limits and valid data subgrouping. **Why Zone A B C Matters** - **Risk Stratification**: Points farther from centerline represent increasing probability of special cause. - **Pattern Detection**: Repeated zone concentration can reveal shift before limit breach. - **Training Simplicity**: Zones make SPC logic easier to teach and apply on the floor. - **Signal Consistency**: Standard zone framework supports comparable rule behavior across tools. - **Decision Speed**: Clear zone-based thresholds accelerate alert classification. **How It Is Used in Practice** - **Control-Chart Setup**: Define centerline and sigma limits from stable baseline data. - **Rule Application**: Apply zone-based run and clustering checks for early anomaly detection. - **Action Differentiation**: Escalate response intensity based on zone severity and persistence. Zone A B C is **a foundational visualization framework in SPC** - zone-based interpretation converts raw variation into actionable control intelligence.