ECONOMIC QUALITY CONTROL OF PRODUCT 



373 



better visualize the extent of these fluctuations in fraction defective by 

 plotting the data as in Fig. 3-a and Fig. 3-b. 



0.04 



0.03 - 



UJ 



> 



o 



UJ 



^. 0.02 



z 

 o 



H 



o 

 < 



DC 



0.01 



I t 



• • • 



. • • • 



0.041- 



a 0.03 



> 



H- 

 O 

 UJ 



u. 



UJ 0.02 



Q 



Z 



o 



1- 

 o 

 < 



0.01 



M 



M 



J J A 



MONTHS 



O 



J L 



M 



M J J A S 



MONTHS 



N 



J L 



Fig. 3 — Should these variations be left to chance? 



a. Apparatus Type A. 



b. Apparatus Type B. 



What we need is some yardstick to detect in such variations any 

 evidence of the presence of assignable causes. Can we find such a 

 25 



