cusum chart

**CUSUM chart** is the **cumulative sum control chart that accumulates deviations from target to amplify detection of small persistent shifts** - it converts subtle bias into visible trend changes for early intervention. **What Is CUSUM chart?** - **Definition**: Chart that sequentially sums signed deviations of observations from a reference value. - **Signal Behavior**: Stable process shows near-flat cumulative path, while shifted process creates sustained slope. - **Sensitivity Profile**: Very strong at detecting small and moderate sustained mean changes. - **Configuration Factors**: Decision interval and reference value determine detection speed and false-alarm rate. **Why CUSUM chart Matters** - **Early Bias Detection**: Captures weak but persistent offsets that Shewhart limits may miss. - **Excursion Prevention**: Enables corrective action before cumulative quality impact becomes significant. - **Diagnostic Clarity**: Slope direction indicates shift direction and persistence. - **High-Value Processes**: Especially useful where small offsets have large yield or reliability impact. - **Continuous Improvement**: Supports tracking of incremental process centering efforts. **How It Is Used in Practice** - **Parameter Calibration**: Tune reference and decision interval using historical process behavior. - **Operational Integration**: Use CUSUM alarms in OCAP with clearly defined escalation steps. - **Dual-Chart Strategy**: Combine with Shewhart charts for broad detection across shift magnitudes. CUSUM chart is **a high-sensitivity SPC method for persistent small-shift control** - cumulative logic provides strong early warning where traditional point-based charts are less responsive.