2 BELL SYSTEM TECHNICAL JOURNAL 



spection and acceptance at one time. These pieces may be parts, partial assem- 

 blies or finished units of product. For purposes of inspection, it is desirable that 

 a lot be composed of pieces all of which have been produced under what are judged 

 to be the same essential conditions. To this end, an attempt should be made 

 to avoid grouping together batches of product that are likely to differ from one 

 another in quality, because of differences in the raw materials used, or differences 

 in manufacturing methods or conditions. For inspections made in a manufac- 

 turing plant, particularly where production is continuous as with conveyor sys- 

 tems, the time element may often be the deciding factor in fixing the size of lot, 

 and such items as convenience in handling, and stocking or shipping facilities 

 may make it desirable to take an hour's, a half-day's, or a day's production as 

 the quantity to be considered as a lot for inspection purposes. 



Quantity Production — Maximum advantage in the use of the tables may be 

 expected for products produced more or less continuously on a quantity 

 basis as distinguished from those produced intermittently on a small 

 scale. 



Inspection by '^Method of Attributes" — Inspection by the "method of 

 attributes"^ is assumed. That is, each piece inspected is examined, gauged, 

 or tested to determine whether it does or does not conform to the re- 

 quirements imposed by specification. 



For some characteristics, the requirements may be expressed as numerical 

 limits to be met by the piece, such as maximum and minimum tolerance limits for 

 a dimension, or the minimum tolerance limit for the illumination of a lamp. 

 For others, the requirements may be expressed in less precise terms, and inspec- 

 tion may consist in observing whether the piece does or does not conform to the 

 finish, appearance, color, etc., of say a standard sample, or to the grade of work- 

 manship commonly understood by the phrase "accepted standards of good 

 workmanship." 



Nondestructive Inspection — The tables are applicable primarily to quality 

 characteristics that may be inspected by nondestructive means, so that 

 at any time it is entirely practicable to inspect every piece in the lot. 



This limitation is a consequence of the inspection procedure adopted in the 

 development of the tables, wherein complete inspection of individual lots is pre- 

 scribed under certain conditions. 



Quality Measured by "Fraction Defective" — The yardstick of quality used 

 in the tables is "fraction defective" (or fraction nonconforming), that is, 

 the ratio of the number of pieces that fail to conform to a specified require- 

 ment to the total number of pieces under consideration. 



A piece of product that fails to meet the requirement for a characteristic is 

 classed as nonconforming with respect to that characteristic, and for convenience 

 is referred to as defective. Thus, a deviation from a specified requirement or 



