CuSum Chart

The CuSum (Cumulative Sum) chart is one of the most sensitive tools in the SPC toolkit. While traditional control charts like Xbar‑R or I‑MR detect large shifts effectively, they can be slow to detect small, sustained changes. CuSum charts excel in this area. They accumulate deviations from the target over time, making even subtle shifts visible long before they appear on other charts. 

The CuSum chart works by calculating the cumulative sum of deviations from the target or mean. If the process remains stable, the cumulative sum fluctuates around zero. If the process shifts—even slightly—the cumulative sum begins to drift upward or downward. This drift provides an early warning signal that the process is changing. 

One of the strengths of CuSum is its sensitivity. It can detect shifts as small as one‑half sigma, which traditional charts may miss for many samples. This makes CuSum ideal for high‑precision processes, regulated industries, or environments where early detection is critical. 

CuSum charts come in two forms: tabular CuSum and V‑mask CuSum. Tabular CuSum uses decision intervals and reference values to determine when a shift has occurred. V‑mask CuSum uses a visual overlay shaped like a “V” to detect deviations. Both methods provide clear, actionable signals. 

However, CuSum charts require careful setup. The target value, reference value, and decision interval must be chosen thoughtfully to balance sensitivity and false alarms. When configured correctly, CuSum provides a powerful early‑warning system. 

In the Control phase, CuSum charts help teams detect small shifts early, maintain tight control, and prevent minor issues from becoming major problems.