Quality Control 



By W. A. SHEWHART 

 Introduction 



A MANUFACTURER is interested in producing a controlled 

 product — one in which the deviations about the average level of 

 quality are no larger than can be accounted for as a result of chance. 

 The present paper gives simple detailed methods for determining from 

 inspection data whether or not a product is being controlled in the 

 sense of indicating the presence of assignable causes of variation. 

 Naturally the inspection data constitutes a sample of the effects of the 

 manufacturing causes and hence the interpretation of these data in 

 terms of what may be expected in the future is a statistical problem. 



A controlled product is defined as one for which the frequency of 

 deviations from the expected quality can be estimated by probability 

 theory. To make such estimates, however, it is necessary to character- 

 ize or specify the distribution of quality which the manufacturer wishes 

 to maintain. These specifications of the desired quality must be 

 arrived at by methods customarily used in setting engineering stand- 

 ards, but when once they have been established the statistical methods 

 amplified in this paper make possible the most economical control of 

 this quality. 



The limits within which quality may be controlled with a given 

 amount of inspection depend upon the standards adopted for the 

 quality to be maintained. 



This paper interprets quality specifications in terms of five different 

 types of constant systems of manufacturing causes. The five types 

 chosen are sufficient to cover the entire range and it is believed that 

 only five types are necessary because sampling theory indicates that 

 little practical advantage would be derived by endeavoring to sub- 

 divide one or more of these. It is shown that quality control can be 

 maintained with the fewest number of measurements and within the 

 closest limits through the adoption of Type V. 



Specification of Control 



One of the principal objects of inspection is the detection of lack of 

 control of manufactured product, that is, the detection of the presence 

 of assignable causes of variation in the quality. A recent paper in this 

 Journal ^ describes a quality control chart designed to attain this ob- 



iShewhart, W. A., "Quality Control Charts," October 1926. 



722 



