StATS: What is a control chart? (November 11, 2006).

A control chart is a graphical tool used in many industrial settings that monitors a work process on a continual and on-going basis. Here is an example of a control chart published in the Engineering Statistics Handbook, published by the U.S. National Institute for Standards and Technology.

Source: www.itl.nist.gov/div898/handbook/pmc/section3/pmc322.htm

There is small typographical error in this chart, but it illustrates the general structure quite well. The control chart is simply a run chart (a plot of a sequence of values) with three reference lines. The center line is typically drawn at the average of all of the data. The control chart also includes two control limits, an upper control limit (UCL) and a lower control limit (LCL). The control limits are set a certain distance away from the center line (I'm deliberately being vague here). Any data values that fall above the UCL or below the LCL are described as "out of control" and represent a "special cause" of variation. If all the data values lie inside the LCL and UCL, the work process is said to be "in control" and all of the observed variation represents "common cause" variation.

This page was written by Steve Simon while working at Children's Mercy Hospital. Although I do not hold the copyright for this material, I am reproducing it here as a service, as it is no longer available on the Children's Mercy Hospital website. Need more information? I have a page with general help resources. You can also browse for pages similar to this one at Category: Control charts, Category: Definitions.