628 BELL SYSTEM TECHNICAL JOURNAL 



modulating device in systems of film sound recording. In this paper 

 the principles of operation of the light valve are discussed, and those 

 engineering factors which prescribe limitations on performance and 

 indicate operating advantages are described in detail. The type of 

 distortion which results when a light valve is overloaded is depicted 

 both for single-plane and two-plane valves. Finally, a new type of 

 light valve having advantages from the standpoints of weight, size, 

 and stability of operation is described. 



Economic Control of Quality of Manufactured Product}^ W. A. 

 Shewhart. This book of 501 pages is an exposition of the technique 

 developed within the Bell Telephone System for securing economic 

 control of quality of manufactured product at every stage in the 

 process of fabrication all the way from raw materials to finished 

 product. It is divided into seven parts, the first of which is devoted 

 to a general survey of the characteristics of a controlled quality, the 

 scientific basis for attaining control, and the economic advantages to 

 be derived. In the second part, after a definition of what is meant by 

 quality, the methods of presenting data are discussed in detail, both 

 graphically and by means of statistics such as the average, standard 

 deviation, skewness, flatness, and correlation coefficient. Part III 

 presents the necessary and sufficient conditions for the specification 

 of a controlled quality. The allowable sampling fluctuations in sta- 

 tistics are indicated in the next part, illustrated by experiments made 

 under conditions known to be controlled. This part constitutes a 

 survey of the present status of sampling theory. Part V takes up 

 the problem of specifying standard quality and indicates the important 

 changes that should be made in many kinds of specifications in order 

 to secure the greatest assurance of uniform quality at minimum cost. 

 Five practical criteria for determining whether or not the quality 

 under consideration differs from standard by more than an amount 

 that should be left to chance are presented in Part VI. In Part VII, 

 after a summary of the fundamental principles, consideration is given 

 to the problem of sampling, and finally a control program is presented 

 which shows the relation of control to research, design, development, 

 production, and purchasing. Several appendices give the original 

 experimental data on which some of this work was based and a bibli- 

 ography. Throughout the book the fundamental principles are amply 

 illustrated by practical examples. 



11 Published by D. Van Nostrand Company, New York, 1931. 



