Learn in the textbookStructured Diagrams for Cause Analysis, Control, and Systematization
DefinitionA control chart (管理図) plots measurements in order as a line and draws a center line plus upper and lower control limits, to judge whether a process is in a stable state.
ExplanationThe horizontal axis shows time order or sample number, the vertical axis shows the quality value, and a center line (CL) marks the average. While the points scatter irregularly between the upper control limit (UCL) and the lower control limit (LCL), the variation is treated as chance and the process is judged stable. A point beyond a limit, or a run of points on one side of the center line, signals a cause that chance can hardly explain, so it gets investigated.
AnalogyLog your morning commute every day and mark the usual range with an upper and a lower line. When a day falls outside that range, check whether there was an accident or roadwork.
Memory hookThe three lines are CL, UCL and LCL: CL stands for Center Line, and UCL and LCL for Upper and Lower Control Limit.