Economic Quality Control of Manufactured Product^ 



By W. A. SHEWHART 



That we cannot make all pieces of a given kind of product identically alike 

 is accepted as a general truth. It follows that the qualities of jjieces of the 

 same kind of product differ among themselves, or, in other words, the quality 

 of product must be expected to vary. The causes of this variability are, in 

 general, unknown. 



The present paper presents a scientific basis for determining when we 

 have gone as far as it is economically feasible to go in eliminating these un- 

 known or chance causes of variability in the quality of a product. When 

 this state has been reached, the product is said to be controlled because it is 

 then possible to set up limits within which the quality may be expected to 

 remain in the future. By securing control, we attain the five economic ad- 

 vantages discussed in Part III. 



I Introduction 



1. What is the Problem of Control? 



WHAT is the problem involved in the control of quality of manu- 

 factured product? To answer this question, let us put our- 

 selves in the position of a manufacturer turning out millions of the same 

 kind of thing every year. Whether it be lead pencils, chewing gum, 

 bars of soap, telephones or automobiles, the problem is much the same. 

 He sets up a standard for the quality of his product and then tries to 

 make all pieces of product conform with this standard. Here his 

 troubles begin. For him standard quality is a bull's-eye, but like a 

 marksman shooting at such a target, he often misses. As is the case in 

 everything we do, unknown or chance causes exert their influence. 

 The problem then is: how much may the quality of a product vary and 

 yet be controlled? In other words, how much variation should we 

 leave to chance? 



To make a thing the way we want to make it is one popular concep- 

 tion of control. We have been trying to do this for a good many years 

 and we see the fruition of this effort in the marvelous industrial develop- 

 ment around us. We have accepted the idea of applying scientific 

 principles but now a change is coming about in the principles them- 

 selves which necessitates a new concept of control. 



A few years ago we were inclined to look forward to the time when a 

 manufacturer would be able to do just what he wanted to do. We 

 shared the enthusiasm of Pope when he said "All chance is but direction 

 thou canst not see," and we looked forward to the time when we would 

 see that direction. In other words, emphasis was laid on the exactness 



^ Paper presented before A. A. A. S. on December 28, 1929, at Des Moines, Iowa. 



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