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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