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154 Part III: Inspection and Test
Average Outgoing Quality
The average outgoing quality (AOQ) is the expected average quality of outgoing
products, including all accepted lots, plus all rejected lots that have been sorted
100 percent and have had all of the nonconforming units replaced by conforming
units.
There is a given AOQ for specific fractions nonconforming of submitted lots
sampled under a given sampling plan. When the fraction nonconforming is very
low, a large majority of the lots will be accepted as submitted. The few lots that
are rejected will be sorted 100 percent and have all nonconforming units replaced
with conforming units. Thus, the AOQ will always be less than the submitted
quality. As the quality of submitted lots declines in relation to the AQL, the percent
of lots rejected increases in proportion to accepted lots. As these rejected lots
are sorted and combined with accepted lots, an AOQ lower than the average fraction
of nonconformances of submitted lots emerges. Therefore, when the level of
quality of incoming lots is good, the AOQ is good; when the incoming quality is
bad and most lots are rejected and sorted, the result is also good.
Average Outgoing Quality Limit
Part III.B
The AOQ is a variable dependent on the quality level of incoming lots. When the
AOQ is plotted for all possible levels of incoming quality, a curve as shown in
Figure 15.2 results. The average outgoing quality limit (AOQL) is the highest value
on the AOQ curve.
Assuming an infinite lot size, the AOQ may be calculated as AOQ = P a p. Probability
of acceptance (P a ) may be obtained from tables as explained earlier and
then multiplied by p.
Lot Size, Sample Size, and Acceptance Number
For any single sampling plan, the plan is completely described by the lot size,
sample size, and acceptance number.
AOQ (%)
4.0
3.6
3.2
2.8
2.4
2.0
1.6
1.2
0.8
0.4
0
0.0367
0.0258
1 2 3 4 5
0.0388
0.0367
N =
0.0312
n = 50
0.0220
c = 3
0.0134
0.0077
10 15 20
p (percent nonconforming)
Figure 15.2 Average outgoing quality curve for: N = , n = 50, c = 3.