50 BELL SYSTEM TECHNICAL JOURNAL 



tive is unchanged. This may result from a change in the definition of a 

 defect; that is, the same condition may at times be considered defective and 

 at other times acceptable. This happens most frequently in border-line 

 cases. To avoid such variations it is necessary that the condition that 

 constitutes a defect be clearly defined and strictly followed in all inspections. 



3.5 The Abnormal Existence of One Kind of Defect 



When the sampHng scheme includes the inspection for several different 

 requirements, the Allowable Number may at times be exceeded because of 

 one kind of defect only. In other words, the lot would be satisfactory if 

 this one kind of defect did not exist. If the same defect persists for several 

 lots, it should receive definite supervisory attention. If substantial im- 

 provement is not feasible, it may be convenient to remove the inspection 

 item to a separate sampling layout. 



3.6 Abnormal Distribution of Defectives 



Occasionally there may be reason to believe that a group of parts sub- 

 mitted for inspection is not uniform in quality throughout, that is, not a 

 true lot as defined in the early part of the paper. Such a group should be 

 divided into homogeneous sections and each section sampled separately. 

 However, when this happens, subsequent lots for sampHng purposes should 

 be similarly subdivided, that is, they should be confined to the output of 

 one source at a time; for example, one machine, one operator, or one batch 

 of material, etc., based on one system of causes, so that control of quality 

 at each source may be applied, and the consumer protected from receiving 

 spotty product. 



In addition to the specific steps followed in estabhshing and operating a 

 sampling plan it must always be remembered that, since relatively impor- 

 tant decisions concerning the acceptance of product hinge upon the results 

 of an examination of a small group of parts, inspection must be conscientious 

 and accurate. In order to guarantee the order of protection promised by 

 the sampling plan, the results of inspection and the prescribed procedure 

 for disposing of individual lots must be regarded with thorough respect. 



