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IRRESPECTIVE of the care taken in defining the production procedure, the manufacturer realizes that he cannot make all units of a given kind of product identical. This is equivalent to assuming the existence of non-assignable causes of variation in quality 2 of product. Of course, random fluctuations in such factors as humidity, temperature, wear and tear of machinery and the psychological and physiologcal conditions of those individuals engaged in carrying out the manufacturing procedure may give rise to some of these apparently uncontrollable variations. Knowing this, the manufacturer contents himself with trying to produce a product which is uniform and controlled — one which does not vary from one period to another by more than an amount which may be accounted for by a system of chance or nonassignable causes producing variations independent of time.
W. A. Shewhart (Fri,) studied this question.