Control Chart

In quality control, a control chart is a graphical display of a quality characteristic that has been measured or computed from a sample versus the sample number or time. The chart contains a center line that represents the average value of the quality characteristic corresponding to the in-control state. Two other horizontal lines, called the upper control limit (UCL) and the lower control limit (LCL), are also drawn. These control limits are chosen so that if the process is in control, nearly all of the sample points will fall between them. As long as the sample points plot within the control limits, the process is assumed to be in control, and no action is necessary. See also statistical quality control and Pareto diagram and cause-and-effect diagram.