© 2004 Cayman Business
Systems
Rev:
Pre-G3 - Rendered Thursday,
February 12, 2004
Slide 44
Elsmar.com --- The
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8-Disciplines Problem Solving
Process Variation
•All variation is caused. There are specific reasons why your weight fluctuates every day, why sales go up, and why Maria performs better than
Robert. Management must recognize that variations
in production or quality within manufacturing or service processes can be viewed as "special cause"
variations, which are best removed by team members operating the process and "common cause"
variations, which require management action to change some inherent feature of the process. There are four main
types of causes.
•Common causes are the myriad of ever-present factors (e.g., process inputs or conditions) that contribute in varying degrees to
relatively small, apparently random shifts in outcomes day after day, week after week, month after
month. The collective effect
of all common causes is often
referred to as system variation because
it defines the amount of variation inherent in the system.
•Special causes are factors that sporadically induce variation over and above that inherent in the system. Frequently, special cause variation
appears as an extreme point or some specific,
identifiable pattern in data. Special causes are often referred to as assignable causes because the variation they produce can be tracked down
and assigned to an identifiable
source. (In contrast, it is usually
difficult, if not impossible, to link common cause variation to any particular source.) Special Cause
variation results from events
which are occurring outside the
process. For example, a relatively major change in temperature or humidity could cause significant variation (points outside control limits) in the
process.