A METHOD OF SAMPLING INSPECTION 617 



to control quality, the immediate objects of individual inspection 

 steps differ. For example, the object of one step may be to secure 

 information which will assist directly in controlling the manufacturing 

 process by detecting errors or trends in performance which would 

 become troublesome if allowed to persist unchecked. In other places 

 the immediate object may be to determine the acceptability of definite 

 quantities of product or to provide a screen for sorting the bad pieces 

 or the bad lots from the good ones. Materials, parts in process and 

 finished units are scrutinized with these objects in view. Depending 

 on circumstances, the character and completeness of inspections vary 

 from visual examination of small samples to careful measurement or 

 testing of each piece. 



Conditions Under Which the Present Method Applies 

 A large amount of industrial inspection work consists in comparing 

 individual pieces with a standard — such as gauging a dimension or 

 measuring an electrical property — to determine whether the pieces do 

 or do not conform with the requirements given in specifications. This 

 is often referred to as inspection by the "method of attributes." Con- 

 sideration will be directed here to the case where non-destructive 

 sampling inspection of this kind is conducted on discrete lots of 

 product for the purpose of determining their acceptability. 



From the standpoint of sampling theory, one of the general require- 

 ments is that each lot should be composed of pieces which were 

 produced under the same essential conditions. Practically, this means 

 that an attempt should be made to avoid grouping together batches 

 of material, which, due to manufacturing conditions or methods, are 

 apt to differ in quality. It is presumed, of course, that the sample 

 drawn from any lot will be a random sample so that it may fairly 

 represent the quality of the entire lot. 



Summing up the general conditions for which a solution is sought, 

 we assume 



(1) The purpose of the inspection is to determine the acceptability 



of individual lots submitted for inspection, i.e., sorting good 

 lots from bad. 



(2) The inspection is made by the "method of attributes" to 



determine conformance with a particular requirement, i.e., 

 each piece does or does not meet the limits specified. 



(3) A lot is homogeneous in quality and the sample from it is a 



random sample. 



The starred items in Fig. 1 indicate the set of conditions involved in 

 our problem. 



