INSPECTION IN MANUFACTURING PLANT 39 



ever, if the results from the first sample are not conclusive, an additional 

 sample is examined before disposition of the lot is made. 



The particular type of articles to which this plan has been applied are 

 products consisting of individual parts, sub-assemblies, or completed ap- 

 paratus, which, at the various stages of production where control is neces- 

 sary, are the result of repetitive operations capable of considerable uni- 

 formit}'. The plan has also been applied to some extent on completed 

 products and purchased materials where there is evidence that the product 

 is of reasonable uniformity even though the quality history is meager or 

 unavailable. 



1. Steps ix Setting up a Double Sampling Lot Inspection Plan 



1.1 Analysis of the Production Process 



In order to determine the applicability of the Double Sampling Plan to 

 existing inspection operations it is necessary to examine the manufacturing 

 and inspection processes and all data available regarding past quality per- 

 formance, such as records of per cent defective, consumers' complaints, etc. 

 The following outline should serve not only as a measuring stick to deter- 

 mine the applicability of Double Sampling but also as an index of the con- 

 ditions to be met for the successful use of Double Sampling with any in- 

 spection operation. 



1.11 The lot, or group of articles to be examined, should consist of 

 product which is available in its entirety for acceptance or rejection at 

 one time. For sampling purposes, the lot should have characteristics 

 which are the result of a common system of causes. By this it is meant 

 that the lot should, as far as possible, consist of articles made from 

 relatively uniform raw material by operators of equivalent skill and by 

 machines or methods of equivalent precision. If there is evidence of 

 appreciable variation between corresponding machines, operators, or 

 materials, it is desirable to confine a lot to the output of one machine, 

 one operator, or one batch of material, in order to isolate a uniform group 

 of product suitable for sampling inspection. 



In brief, Double Sampling may be applied to the output of any repeti- 

 tive unit operation capable of sufficient uniformity. However, unless 

 immediately essential for economic reasons, it need not be applied at the 

 particular time such an operation is completed, provided succeeding 

 operations do not modify the inspection item under consideration. 



1.12 In order to gain the maximum advantage from the use of Double 

 Sampling, it is necessary that lots be as large as the limitation of uni- 

 formity will allow so that protection and control may be achieved with 



