Control chart

Control charts (also known as Shewhart charts, after Walter A. Shewhart, or process-behavior charts) are graphical plots used in statistical process control (SPC) to determine whether a manufacturing or business process is in a state of statistical control. They form the basis of modern statistical process control, and are one of the seven basic tools of quality control.

A control chart plots a process statistic, such as a mean, range, or proportion, over time together with a centre line and upper and lower control limits that distinguish common-cause variation from special-cause variation. Points outside the control limits or non-random patterns within them indicate that the process should be investigated for assignable causes of variation. When a process is in statistical control, control charts can be used to predict its future performance. Control charts were invented by Shewhart at Bell Labs in the 1920s.