Causes of Variation  Two basic categories of variation in output include common causes and assignable causes.  Common causes are the purely random, unidentifiable sources of variation that are unavoidable with the current process.  If process variability results solely from common causes of variation, a typical assumption is that the distribution is symmetric, with most observations near the center.  Assignable causes of variation are any variationcausing factors that can be identified and eliminated, such as a machine needing repair. © 2007 Pearson Education
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