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Control Charts

When measuring real data there is one central point which should never be overlooked: all processes have some variation. This variation result from various differences in the production process (e.g. different suppliers of raw material, different calibrations, temperature dependencies, etc). Normally, one would put most effort into controlling the variability of a process. Nevertheless, there will still be some remaining variation in the final product which cannot be eliminated. This variation has to be controlled in order to check the stability of a process.

There are several possibilities to monitor, or control, a process by control charts. These charts were introduced in the late 1920s by Shewhart, and display simple measures of location and variation against time. In addition, a control chart also displays pre-defined limits which should not be exceeded by stable processes.

To be specific, there are four major types of control charts, which are most often used in process control:

Last Update: 2005-Jul-16