Introduction to Control Charts

Uncontrolled variation is the enemy of quality.

—W. Edwards Deming

A control chart is defined as a “chart with upper (UCL) and lower (LCL) control limits on which values of some statistical measure for a series of samples or subgroups are plotted. The chart frequently shows a central line (CL) to help detect a trend of plotted values toward either control limit.”1 Note the absence of any mention of specification limits in this definition. Specifications represent the organization “talking” to the process—that is, telling the process what the organizations desires. A control chart represents “the process talking to the organization”—telling the organization what the process can do. Specifications should ...

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