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BELL SYSTEM TECHNICAL JOURNAL 



by week or month by month will fluctuate around some average value 

 according to the laws of chance. For example, in the manufacture of 

 selectors assume conditions are such as to give uniform quality with an 

 expected rate of 0. The rates for batches of selectors turned out 

 weekly will fluctuate about Rate =0, the range of variation depending 

 on the number produced each week. A week's output can be regarded 

 merely as a sample of the product which this system of causes would 

 turn out if it were allowed to function in the same manner for an 



Protatility of Occurrence 

 of Different Values of 

 Rate 



Probability of Occurrence 

 of Different Values of 

 .Bate 



Weekly Output of 

 1000 



Units Each 



-10 oj -10 



Samples of 



:jOC Units 



Each 



Fig. 4 — Typical fluctuations of rates for uniform product whose expected rate = 



indefinite length of time. The distribution of weekly rates is the same 

 as would be obtained in an ordinary sampling experiment by drawing 

 samples from an infinite warehouse of thoroughly mixed selectors 

 having an average quality represented by Rate = 0. If inspection 

 consists in examining only a percentage of the selectors manufactured, 

 this will be merely equivalent to taking smaller quantities of selectors 



