Control Chart

Posted on February 17, 2009 by

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The Control Chart is a type of run chart that shows whether a process is in statistical control by adding lines called Control Limits to the chart. There is both an Upper Control Limit and a Lower Control Limit. It is also common to add a line representing the mean (average), and sometimes lines representing specification limits. The Control Limits are frequently drawn three standard deviations above and three standard deviations below the mean, because normally over 99% of the data should fall between these two limits. When the data exceeds these limits, it is worth investigating. There are different varieties of Control Charts named for the type of data it contains (c-chart for count type data, p-chart for proportions, etc.) that are especially common in Six Sigma. However, the underlying principle is the same.

There are three Quality processes described in the fourth edition PMBOK®—Plan Quality, Perform Quality Assurance, and Perform Quality Control. Plan Quality and Perform Quality Control specifically list the Control Chart as a tool and technique, and Perform Quality Assurance contains all the tools and techniques of the other two processes, so the Control Chart is a part of the all quality processes.

Below is an example of a Control Chart showing how many cookies Katie ate each day for three weeks. The average number of cookies she eats a day is 9, which is drawn with the red line. The blue line is the Upper Control Limit (which is 3 standard deviations above the mean) and the purple line is the Lower Control Limit (which is 3 standard deviations below the mean). What the Control Limits make clear is that on day 10, Katie enjoyed eating an exceptional number of cookies!

Control Chart

Control Chart