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BELL SYSTEM TECHNICAL JOURNAL 



should fall within well-defined limits such as indicated on the chart. 

 Reference to the x^-part of the control chart, Fig. 4, shows that this 

 test gives more conclusive evidence than any other for deciding that 

 the product had not been controlled. As previously noted, further in- 

 vestigation revealed the assignable causes of lack of control. This is a 



Fig. 4 



common experience under such circumstances. Furthermore, it is of 

 interest to note that the preparation of such a chart requires but a 

 small amount of labor on the part of a computer. 



Discussion and Conclusion 



This paper shows how statistical methods may be used to detect lack 

 of control of product. It describes a recently developed form of manu- 

 facturing control chart which helps in the use of inspection and pro- 



