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A source of *Quality* failure that is always present as part of the random *Variation* inherent in the *Process* itself.
Its origin can usually be traced to an element of the process which only management can correct.
The less well-defined a process is, the more it is subject to random variation, resulting in a higher level of quality failures (bugs).
In general, and very approximately, Common Causes outweigh *Special Causes* as origins of quality failures by four to 1 (*Pareto* distribution).