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.

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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, 360 Difference, and now too.
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