Downloaded from https://www.everyspec.com submitted. For example, if 1,000 lots of aqual- poorest average quality that the plan will ac- ity 6 percent defective are submitted, then under cept. Obviously, the quality of the accepted or the sampling plan indicated by the OC curve in, outgoing lots tan never be worse than that of figure 5-1, 620 lots will be accepted; their av- the incoming or submitted lots. erage quality will be 6 percent defective. If from the OC curve a curve of AOQ values is 5-7.
Establishing the Acceptable Quality Level plotted, it will be found to be a straight line, for The ideal goal of procurement is to ac- the average outgoing (accepted) quality is always cept only perfect materiel. However, several equal to the average incoming (submitted) qual- factors involved in the attainment of such per- ity. This is true in the case of destructive teut- fection make it necessary that something less ing when rejected lots are scrapped. than perfection be accepted.
First of all, if a manufacturer were forced to submit perfect materiel at all times, his manufacturing costs However, when rejected lots are screened 100 would be very high, and the resultant cost would percent and resubmitted for inspection, the AOQ be high. Secondly, to be assured of buying only of the accepted lots will be better than the av- perfect materiel, 100 percent inspection on all erage incoming quality. For example, if 1,000 lots submitted for acceptance would have to be lots of a quality 6 percent defective are inspect- made. This would make the cost of inspection ed under the plan indicated in figure 5-1, 620 lotswil beaccpte an 380lot reectd. l.. prohibitive.
To maintain manufacturing expense Slots will be accepted and 380 lots rejected. Af- and cost of inspection at a reasonable level, less ter the rejected lots are screened and finally than perfect materiel must be accepted. The accepted, the quality of these lots will be 0 per- question then arises: at what point will the cent defective. The AOQ for the total lots will then be combined cost of inspection and manufacture be reasonable, while still primarily assuring the acceptance of good materiel? This question 6203.72 percent should be answered whenever an AQL is set.
1000 The AQL may not be the perfect answer in all which is better than the incoming (submitted) cases, however, because of the numerous fac- which, i ertors which must be taken into account when es- tablishing an AQL. If from the OC curve (figure 5-1) the AOQ val- In establishing an AQL the most important con- ues for the range of qualities are computed and plotted, the AOQ curve will be similar to that sideration is the serioxisneqs of the defect. The shown in figure 5-2.
It will be noted that there degree of compromise made with respect to the Is a maximum value known as the Average Out- quality considered acceptable is completely de- going Quality Limit (AOQL), which is the pendent upon this factor. Systems of classify- ing defects assist in permitting defects of simi- lar natures to be treated alike. 0-- - - - 7 One of the factors to be considered when estab- I- _lishing an AQL is the degree of manufacturing " -difficulty associated w.th the item. Cost is in- U2 - - - -.- _ directly involved in this consideration, as costs increase with greater manufacturing difficulty.
To determine the degree of manufacturing diffi- " '...o0 culty for a particular item, any knowledge of - - -actual expe,' nce with the item should be put to use. For ,urposes of setting the AQL in those cases where no data exists, the degree of o omanufacturing difficulty is associated with the 0No . A.0 4.o number of defects listed under one classifica- INC OMINt G QUALITY (PENCEINT DEFECTIVE) tion. It is obviously more difficult to control five characteristics than one, therefore the AQL Figure 5-2. Typical curte of Average would be less stringent for five than for one. Outgoing Quality (AOQ) Assuming the defects are independent of each 5-4 4