Page 232 - DMGT501_OPERATIONS_MANAGEMENT

Operations Management

Notes There are two widely used ways of picking (n,c):
1. Use tables (such as MIL STD 105D) that focus on either the AQL or the LTPD desired.
2. Specify 2 desired points on the OC curve and solve for the (n,c) that uniquely determines
an OC curve going through these points.

8.5 Summary

Sampling plans are typically set up with reference to an acceptable quality level, or AQL.
The AQL is the base line requirement for the quality of the producer’s product.
The producer would like to design a sampling plan such that the OC curve yields a high
probability of acceptance at the AQL.
On the other side of the OC curve, the consumer wishes to be protected from accepting
poor quality from the producer. So the consumer establishes a criterion, the lot tolerance
percent defective or LTPD.
Here the idea is to only accept poor quality product with a very low probability. Mil.
Std. plans have been used for over 50 years to achieve these goals. SPC is preferred over
Acceptance Sampling because it provides near real­time monitoring of the process.
Acceptance sampling ignores the process and focuses exclusively on the output after it has
been produced.
8.6 Keywords

Acceptable Quality Level (AQL): The AQL is a percent defective that is the base line requirement
for the quality of the producer’s product.

Average Outgoing Quality (AOQ): A common procedure, when sampling and testing is non­
destructive, is to 100% inspect rejected lots and replace all defectives with good units.

Lot Tolerance Percent Defective (LTPD): The LTPD is a designated high defect level that would
be unacceptable to the consumer. The consumer would like the sampling plan to have a low
probability of accepting a lot with a defect level as high as the LTPD.
Multiple Sampling Plans: This is an extension of the double sampling plans where more than
two samples are needed to reach a conclusion. The advantage of multiple sampling is smaller
sample sizes.

Operating Characteristic (OC) Curve: This curve plots the probability of accepting the lot
(Y­axis) versus the lot fraction or percent defectives (X­axis). The OC curve is the primary tool for
displaying and investigating the properties of a LASP.
Sequential Sampling Plans: This is the ultimate extension of multiple sampling where items are
selected from a lot one at a time and after inspection of each item a decision is made to accept or
reject the lot or select another unit.
Skip lot Sampling Plans: Skip lot sampling means that only a fraction of the submitted lots are
inspected.

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