Variation is the differences in results obtained in measuring the same phenomenon more than once. The sources of variation in a process over time can be grouped into two major classes: common causes and special causes. Excessive variation frequently leads to waste and loss, such as the occurrence of undesirable patient health outcomes and increased cost of health services. Common-cause variation, also called endogenous cause variation or systemic cause variation, in a process is due to the process itself and is produced by interactions of variables of that process is inherent in all processes, not a disturbance in the process. It can be removed only by making basic changes in the process. Special-cause variation, also called exogenous-cause variation or extra systemic cause variation, in performance results from assignable causes. Special-cause variation is intermittent, unpredictable, and unstable. It is not inherently present in a system; rather, it arises from causes that are not part of the system as designed.

J. Jerrald Hayes
I am ex-architectural woodworker and general contractor turned IT, Business and Project Management consultant, software developer wannabe senior division triathlete and ski racer, Yankee fan and founder of Paradigm-360.com, 360 Difference Mac4Construction.com,iOS4Construction.com and now TheBuildingAndRemodelingWiki.com too.
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