Common and Special Cause Variation
Common Cause Variation
Common cause variability occurs naturally in every process. Common cause
variation is fluctuation caused by unknown factors resulting in a steady
but random distribution of output around the average of the data. This
fluctuation defines process potential, or how well the process can
perform when all special cause variation is removed.
Common cause variation is also called random variation or noise. Example: Many small variations with a small impact. Common cause variation is the remaining variation after removing the special causes (non-normal causes) due to one or more of the 6Ms (Man power, Mother nature, Materials, Method, Measurements or Machine). Common cause variation is measured with control charts as a fundamental metric of quality improvement.
Special Cause Variation
Special cause variability is also unavoidable
in most every process. Special cause variation is caused by known factors
that result in a non-random disruption of output. Sometimes referred
to as "exceptional" or "assignable" variation. Example:
earthquake, and other environmental causes or catastrophic mechanical
failure. Special cause variation can be thought of as the few with major
impact. Special cause variation
can be accounted for directly and potentially removed. Measured continuously,
identified and removed through the proper use of control
charts we
take this as a measure of process control.
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