What is a control chart and what do the control limits mean?

A control chart plots process measurements over time against statistically calculated upper and lower control limits, typically set at three standard deviations from the mean. Points within the control limits indicate normal process variation. A point outside the control limits signals that the process is out of control and requires investigation. The rule of seven states that seven consecutive points on one side of the mean, even within control limits, indicate a non-random trend that should be investigated.