978 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1953 



the various control limits for factory use is a relatively simple operation. 

 A typical example is the 1507 A Inductor where the inductance require- 

 ment is expressed as 283.9 =b 20 microhenries. 



Then the "C" limits = 283.9 =t 0.5 X 20, = 293.9 max., 273.9 min. 

 microhenries, and the "E" limit = 1.48 X 20 = 29.6 microhenries. 



Since the distribution requirements are a part of the specification, the 

 product must meet rather exact criteria in terms of the limiting condi- 

 tions previously described, even though they may appear complicated 

 in terms of the usual manufacturing procedures. In the case of the Con- 

 tinuous production method the criteria are applied to the samples of 

 5 units each, in two steps, (1) to establish eligibility, either at the start 

 of production or when eligibility is lost and (2) to maintain eligibility 

 when once established. Prior to the establishment of eligibility or when 

 eligibility is lost the three-cell method is applied. 



Criterion 1 



Eligibility is established as soon as 7 consecutive samples of 5 units 

 satisfy the following: 



(a) The averages, X, all fall within the C limits. 



(b) The ranges, R, all fall below the E limit. 



(c) Seven consecutive averages, X, are not all outside the same D 

 limit (not all above the upper D limit or all below the lower D limit.) 



Criterion 2 



Eligibility is maintained as long as each current sample of 5 units 

 satisfies the following: 



(a) The average, X, either 



1. falls within the C limits or 



2. falls outside the C limits but at the same time all of the 6 

 preceding consecutive averages fall within the C limits. 



(b) The range R either 



1. falls within the E limit or 



2. falls outside the E limit but at the same time all of the 6 pre- 

 ceding consecutive ranges fall within the E limit. 



(c) Seven consecutive averages X (the current sample and the six 

 preceding samples) do not fall outside the same D limit. 



2.32 Batch Production Method 



This method is for application where production consists of intermit- 

 tent batches of 50 or more units, all of which have been made under the 



