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Common causes
of variation result from such things as system design decisions and the use of one
development tool over another. This variation will occur predictably across the
entire process associated with it and is considered normal variation. Special causes
of variation are those that arise from people-type errors and unplanned events. This
Statistical Process Control n 237
variation is exceptional variation. Other terms you will hear are in control for predictable
processes or steady-state; and out of control for unpredictable processes that
are ???outside the natural limits.???
When a process is predictable, the process exhibits routine variation as a result of
common causes. When a process is unpredictable, it exhibits exceptional variation
as a result of assignable causes. It is our job to be able to tell the difference and to
find the assignable cause. When a process is predictable, it is performing as consistently
as it can (either for better or for worse). It will not be performing ???perfectly???;
there will always be some normal, routine variation.


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