954 



THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1953 



-0.4A -0.2A 0.2A 0.4A 



DEVIATION FROM NOMINAL 



Fig. 3 — Basis of A5 limits and PA limits for sample averages. 



customary upper 3-sigma control limit for ranges, D2(j" (where Di = 

 4.918 for samples of 5). Since the relation between the specified limits 

 for individual units and cr'' is expressed by A = 3^ a-'', the above limits 

 are related to the specification limits as follows : 



PA limits = iV ± O.IA 



A5 limits 

 R5 limits 



N =b 0.5A 

 lASA, max. 



At the outset and whenever eligibility to use the control chart method 

 is lost, a 100 per cent inspection rate is required and acceptance is based 

 on the three-cell method (discussed later) . Units in groups of 5 are tested 

 as successive samples, and subjected to the following criterion: 



Criterion I — Establishing Eligibility. 



Eligibility for use of the control chart method is established as 

 soon as 7 consecutive samples or groups of 5 satisfy all the fol- 

 lowing conditions: 



(a) The averages all meet the A5 limits; and 



(b) The ranges all meet the R5 limit; and 



