40 BELL SYSTEM TECHNICAL JOURNAL 



a minimum sample size relative to the number of units in the lot. The 

 fact that proportionately smaller sample sizes are used with the larger 

 lot sizes may be seen by reference to the Sampling Table of Fig. 1. 



In sampling from larger lots it becomes increasingly important in 

 practice to observe certain precautions in order to take care of instances 

 where the lot may not be homogeneous; specifically, each sample should 

 be a group of articles taken at random from different locations throughout 

 the lot so that it will represent an impartial cross section of the lot. 



1.13 It is necessary for the successful operation of any sampling in- 

 spection that at all times there be a known standard of acceptability for 

 the individual article, that is, a definite description of the requirement 

 for each inspection item and reliable measuring equipment against which 

 product may be conclusively checked. Practically, this condition will 

 be realized when the characteristic is defined in such a manner that differ- 

 ent observers will obtain consistent results. 



1.14 The theoretical background of the plan assumes the repair or 

 elimination of all defectives- in samples of accepted lots, as well as all 

 defectives in rejected lots before such lots are passed. 



In order to accomplish this, close cooperation between production and 

 inspection personnel is required in assuring that rejected lots are thor- 

 oughly cleared of all defectives, since failure to comply with this funda- 

 mental part of the procedure destroys the very foundation of the plan. 



1.2 Selection of Proper Double Sampling Table 



Tables are provided for a variety of Average Outgoing QuaUty Limit (AO 

 QL) values and Process Average classes. The AOQL value is the maximum 

 value of average per cent defective in the product after inspection, which the 

 sampling plan will assure over a long period of time, no matter how defective 

 the product submitted for inspection may be. The process average is the 

 normal per cent defective which is to be expected from the process. 



To determine what AOQL valve should be adopted, it is necessary to 

 decide upon a maximum average per cent defective which may be permitted 

 in the product without serious consequences to the user. Product which is 

 of such a nature that defects will be eliminated in subsequent operations 

 may be assigned a rather generous AOQL, and conversely, product which 

 by its relation to the entire assembly may cause considerable inconvenience 



- A defective is defined as an individual article that fails to meet the requirements for 

 one or more inspection items. A defect, however, is defined as a failure to meet a require- 

 ment for a single quality characteristic for which inspection is made. It follows that when 

 several characteristics are inspected, an individual article may have several defects, yet 

 be only a single defective. 



