Definition
Data collection tools to show whether the process is within established control (controlled).
In detail
A control chart is a statistical tool used to monitor process stability. By plotting data points, a center line, and upper and lower control limits, it helps determine whether a process is in a state of control. In the system, it addresses the problem of distinguishing between common cause variation and special cause variation, guiding whether to take intervention actions. Compared to a run chart, a control chart adds statistical control limits, enabling more objective anomaly detection; compared to a histogram, it focuses on time order. Typical output is a chart with control limits, used for continuous monitoring and anomaly alerting.
Example in practice
An injection molding workshop finds that product dimensions fluctuate considerably, but cannot determine whether this is random variation or caused by equipment aging. The engineer first uses 20–25 groups of historical data (each group 4–5 pieces) to calculate control limits, then plots points daily and continuously records the first-piece dimensions; one day a point exceeds the control limit, and a special cause is judged to exist. Investigation then reveals mold wear; after replacement, the Control Chart returns to normal and the process is back in control.