368 BELL SYSTEM TECHNICAL JOURNAL 



The control limits for the rate are obtained from the equation 



(assuming the Normal Law to be a satisfactory approximation for 

 determining probabilities), where an^ is given by equations (2) and (6), 

 and where K is a constant whose value depends on the choice of 

 probability. 



The following table gives values of K for different values of probabil- 

 ity. 



Probability -f^ 



.997 3.00 



.990 2.33 



.955 2.00 



.900 1-65 



.800 1.28 



.683 1.00 



,500 675 



