zone a spc
**Two Points in Zone A** is an SPC (Statistical Process Control) warning signal indicating potential process instability when two out of three consecutive points fall in the outer thirds of a control chart.
## What Is Zone A?
- **Location**: Between 2σ and 3σ from centerline (both sides)
- **Rule**: 2 of 3 consecutive points in Zone A triggers warning
- **Type**: Western Electric Run Rule (WE Rule 2)
- **Action**: Investigate process, don't wait for out-of-control
## Why This Signal Matters
Points in Zone A are statistically unusual but not yet out-of-control. Two in quick succession suggests a process shift is developing.
```
Control Chart Zones:
━━━━━━━ UCL (3σ) ━━━━━━━━
Zone A (2σ to 3σ) ← Warning zone
━━━━━━━ (2σ) ━━━━━━━━━━━
Zone B (1σ to 2σ)
━━━━━━━ (1σ) ━━━━━━━━━━━
Zone C (within 1σ) ← Normal variation
━━━━━━━ CL (mean) ━━━━━━
```
**Response Protocol**:
1. Document the occurrence
2. Check for special cause variation
3. Review recent process changes
4. Monitor next several points closely