672 BELL SYSTEM TECHNICAL JOURNAL 



is of much broader interest. The economic control of quaHty of 

 manufactured goods is perhaps the simplest type of scientific control. 

 Recent studies in this field throw light on such broad questions as: 

 How far can Man go in controlling his physical environment? How 

 does this depend upon the human factor of intelligence and how upon 

 the element of chance? 



Observational Significance of Accuracy and Precision.^ W. A. 

 Shewhart. Two of the most common terms used in pure and applied 

 science are accuracy and precision. When such terms are used, as in 

 the specification of quality of manufactured products, it is desirable 

 that they have definite and, in so far as possible, experimentally veri- 

 fiable meanings. It is, therefore, important to determine how far one 

 can go towards attaining this end by applying with rigor the principle 

 that only that which is observable is significant. In the application 

 of the concepts of accuracy and precision, it is customarily assumed 

 that the available data constitute a random sample. Hence, the first 

 step in attaining experimentally verifiable meaning of these terms is to 

 choose an operationally verifiable criterion of randomness. One such 

 criterion is the quality control chart. In order to give experimental 

 definiteness to any measure of either accuracy or precision derived 

 from a random sample, it is also necessary to specify the way any 

 statement involving the measure may be experimentally verified. To 

 do this it is necessary to make at least four empirical choices as to the 

 details of taking and analyzing the data in the process of verification. 

 Hence, it appears that the meaning of either precision or accuracy is 

 verifiable. Hence, it appears that the meaning of either precision or 

 accuracy is verifiable only in a limited sense subject in any specific case 

 to the choice of empirical criteria of verification. 



The Time Lag in Gas-Filled Photoelectric Cells. ^ A. M. Skellett. 

 In commercial gas-filled photoelectric cells there is a lag in response 

 which becomes appreciable above frequencies in the neighborhood of 

 10,000 cycles. If this lag is due to the transit times of the ions across 

 the cell, it should be possible to set up resonance conditions by varying 

 the frequency of modulation of the incident light intensity. This has 

 been accomplished in a cell of special design and the resonance condi- 

 tions agree with the theory, thereby demonstrating that the transit 

 time of the ions is the simple cause. The paper also discusses the flow 

 of the ions and electrons across the cell and their impacts in relation 

 to the flow of current in the external circuit. 



5 Jour. Wash. Acad. Sciences, August 15, 1938 (p. 381). 



^ Internal' I. Projectionist, September 1938; Jojtr. Applied Physics, October 1938. 



