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Statistical Process Control

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A branch of quality control and improvement that employs graphical methods to monitor and improve processes. The central idea of SPC is that all processes vary over time and that the process variation can be broadly categorized into common cause and special cause variation. Common cause variation is the background or ‘noise’ variation which is always present at any given time. Special cause variation on the other hand occurs due to a shift or break in the process which is not normal to the process and the source of which need to be investigated and eliminated.

In order to distinguish between special and common cause variation, SPC practitioners use a tool called a control chart. Process data collected over time are placed on this chart, which features upper and lower control ‘limits’ derived from the data. Several criteria have been proposed to evaluate whether or not the process is stable, or ‘in control’. If any of the criteria are failed, the chart ‘signals’ an out-of control condition indicating the presence of a special cause. The information from these charts is also used to assess process capability.


Different types of control charts are available depending on the type of process data being collected. These are listed in the table shown.

External Links's Toolbox - SPC - SPC Overview - American Society for Quality (ASQ) - Developing Control Charts and Illustrating Type I and Type II Errors by Elisabeth J. Umble and M. Michael Umble from ASQ -