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.