| In Statistical
Process Control, the variation
in a process is categorized as either Common
(General) Cause Variation or Special (Assignable)
Cause Variation.
The common cause variation
arises from a multitude of small factors
that invariably effect any process and will
conform to a normal distribution, or a distribution
that is closely related to the normal
distribution. The special cause
variation arises from specific factors that
have an identifiable effect on the process.
Common cause variation is inherent in the
process and can only be reduced by changes
to the system, and usually requires management
action eg. upgrading a machine. Special
causes can often be tracked down and fixed
without extensive changes to the system
eg. fixing a fault on a machine or improving
the quality of raw materials.
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