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.