shift detection

**Shift detection** is the **identification of sudden and sustained changes in process centerline level** - it distinguishes true step changes from normal random variation in SPC data. **What Is Shift detection?** - **Definition**: Detection of abrupt mean displacement where subsequent points stabilize around a new level. - **Signal Pattern**: Commonly appears as runs on one side of centerline or repeated zone-rule violations. - **Potential Causes**: Component replacement, recipe update, material lot change, calibration error, or operator adjustment. - **Analytical Basis**: Uses run rules, zone rules, and change-point style comparisons. **Why Shift detection Matters** - **Rapid Containment**: Early shift recognition limits lot exposure before defect impact grows. - **Root-Cause Precision**: Step-change timing helps correlate to specific events in maintenance or operations logs. - **Capability Protection**: Uncorrected shifts erode Cpk even when short-term variation is unchanged. - **Quality Governance**: Shift events require formal response under SPC control plans. - **Operational Confidence**: Distinguishes normal noise from true process-state change. **How It Is Used in Practice** - **Event Correlation**: Match shift onset to maintenance actions, lot transitions, and recipe revisions. - **Response Standards**: Use OCAP workflows with hold, verify, and controlled restart criteria. - **Post-Fix Verification**: Confirm centerline restoration with stable follow-up data. Shift detection is **a high-value SPC capability for step-change control** - fast identification and disciplined response prevent localized events from becoming broad production excursions.