374 



BELL SYSTEM TECHNICAL JOURNAL 



yardstick? Experience of the kind soon to be considered indicates an 

 affirmative answer. It leads us to conclude that it is feasible to es- 

 tablish criteria useful in detecting the presence of assignable causes of 



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Fig. 4 — Should these variations be left to chance? 



a. Xo. 



b. Yes. 



variation or, in other words, criteria which when applied to a set of 

 observed values will indicate whether or not it is reasonable to believe 



