long-term drift

**Long-term drift** is the **gradual movement of process or equipment output over extended time due to wear, aging, and condition change** - it is a slow special-cause pattern that can erode capability before hard alarms occur. **What Is Long-term drift?** - **Definition**: Progressive baseline shift in key parameters across weeks or months. - **Primary Drivers**: Component aging, contamination buildup, calibration offset growth, and environmental change. - **Observed Signals**: Mean movement, increasing correction demand, and recurring near-limit excursions. - **Detection Approach**: Trend analytics and periodic baseline comparisons rather than point-only checks. **Why Long-term drift Matters** - **Capability Erosion**: Slow center shift can reduce margin and increase defect sensitivity. - **Hidden Risk**: Drift may stay within limits for long periods while quality robustness declines. - **Maintenance Timing**: Drift trends provide early indicator for planned intervention. - **Yield Protection**: Early correction avoids broad excursion events later. - **Asset Strategy**: Persistent drift informs refurbishment or replacement decisions. **How It Is Used in Practice** - **Trend Monitoring**: Track long-window means and slopes for critical process and equipment signals. - **Baseline Refresh**: Compare current state to qualified reference after controlled intervals. - **Preventive Actions**: Schedule recalibration, cleaning, or component replacement before limit crossing. Long-term drift is **a major slow-failure mechanism in manufacturing systems** - managing drift proactively is essential for sustained process capability and predictable yield.