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BELL SYSTEM TECHNICAL JOURNAL 



relatively simple products. The number of classes that can logically 

 be used in any case depends upon the accuracy which can be attained 

 in making estimates of relative seriousness. 



Before proceeding further it may be well to indicate how the defects 

 for features which are inspected as "variables" are weighted. Take 

 the illustration accompanying Fig. 1. Any failure to meet the com- 

 mercial limits of .099 and .101 microfarad will result in irregularities in 

 transmission such as the distortion of the words spoken over a telephone 

 line. The greater the departure from these limits the greater is the 

 seriousness from a service standpoint. Strictly the weight for a defect 

 should depend upon the degree of its departure from a limit but the 

 desired result can be approximated to a satisfactory degree of accuracy 

 by classifying the defects into two or more classes. To illustrate, 

 assume two classes as indicated in Fig. 2. Defects falling within the 



Fig. 



Capacity (in microfarads) 

 2 — Classification of defects for variable characteristics 



ranges .098 to .099 and .101 to .102 are serious and can be considered 

 as Class "B" defects in a four-fold classification while defects outside 

 of the two outer limits .098 and .102 are Class "A" defects and can be 

 weighted as such. 



Computation of the Rate 



A defect is weighted by assigning to it a number of "demerits." 

 For a given kind of product each class of defects has a specified weight. 

 Since the relative weights are alone of importance, the scale of demerits 

 may be chosen arbitrarily. 



The unit of measurement in the rating plan is "demerits per unit." ^ 

 This factor is the simple sum of the demerits per unit contributed by 

 the different types of defects found in inspection. 



2 The "unit" is commonly a physical unit of product such as a piece part, a partial 

 assembly or a finished unit of apparatus or equipment. Exceptions to this rule have 

 been found desirable for certain complicated types of product, such as switchboard 

 sections ori nstalled central office equipment, in which cases the unit may be a natural 

 element ol a physical unit such as a soldered connection, a circuit, etc. 



