INSPECTION IN MANUFACTURING PLANT 49 



factory. However, it should be determined whether or not succeeding 

 operations can possibly tolerate more defectives than they are actually 

 receiving and, if this is the case, the AOQL may profitably be increased. 



3.2 Changes in Process Average 



The economy of the sampling plan tends to decrease when the level of 

 quality of the product shifts outside the range of process average on which 

 the layout was based. The sampling table on the layout should ordinarily 

 be changed whenever the process average shifts from one range to another. 

 However, before reissuing the layout, the reason for the shift should be 

 determined. If the process average has been reduced, an attempt should 

 be made to make the change permanent; or if it has been increased, the 

 cause should be eliminated. 



3>2 Interpretation of the Control Chart on ^^ Summary of Results of Inspection'^ 

 Form 



It is very important to review the Control Chart of per cent defective (Fig. 

 4) frequently as it is an index of the success of the sampling plan. Not all 

 fluctuations of the per cent defective are significant. Even though the 

 quality of the product is controlled, the results of sampling inspection may 

 produce fluctuations in the indicated per cent defective. These variations 

 are measured by a simple control chart on the ''Summary" form which em- 

 ploys control limits for values of per cent defective. As long as the plotted 

 points of the per cent defective remain within control limits the fluctuations 

 are no greater than may reasonably be expected from a uniform manufac- 

 turing process. However, if a point goes above the upper control limit, 

 the cause may be defective workmanship, defective raw materials, or even 

 a change in the severity of inspection. If a point goes below the lower con- 

 trol limit, the cause may be an improvement in quality or a change in the 

 definition of a defect through misinterpretation or changes in inspection 

 equipment or method of check. If the curve of the plotted points hugs 

 either limit line or shows a definite trend toward one side or the other, a 

 significant change in the quality of the product is indicated. 



In order to achieve control of the quality of a manufactured product, 

 direct and immediate action must be taken in order to stem unfavorable 

 trends. The presence or absence of a satisfactory quality level may be 

 detected by means of inspection, but such a level can only be originated and 

 maintained by adequate manufacturing methods and equipment in the 

 hands of a quality-minded producing personnel. 



3.4 Changes in the Definition of a Defect 



Either laxity or severity of inspection may cause the "reported" per cent 

 defective to show significant changes even though the actual per cent defec- 



