- Features of ANSI/ASQ Z1.4 including the inspection level, AQL, sample size n, and acceptance number c, along with the switching rules prescribed by the standard.
- Producer’s risk of rejecting a lot at the AQL
- The operating characteristic (OC) curve, which reflects the chance of accepting a lot whose nonconforming fraction is p. This allows comparison of the c=0 plan to the traditional plan.
- The c=0 plan as a form of discovery sampling, which calculates the necessary sample size (n) to be a certain percent—in this case, 90%–certain to detect at least one nonconformance if the problem is present at a certain level (in this case, the rejectable quality level).
- The need for quality to far exceed the AQL due to the higher producer’s risk of rejection of lots at the AQL by c=0 plans. Alternatives such as sequential sampling and narrow limit gauging, both of which reduce the amount of necessary inspection, are available when this requirement is not met.
Traditional ANSI/ASQ Z1.4 (formerly MIL-STD 105) sampling plans specify a sample size n and acceptance number c. Double and multiple sampling plans reduce the average sample number (ASN) by accepting very good lots on the first sample, and similarly rejecting very bad lots. Any ANSI/ASQ Z1.4 plan can however be converted into a zero acceptance number sampling plan (c=0 plan) that minimizes the amount of necessary inspection even further. Use of c=0 plans requires, however, that outgoing quality far exceed the specified acceptable quality level (AQL) due to the increased risk of rejecting lots at the AQL.
William A. Levinson, P.E., is the principal of Levinson Productivity Systems, P.C. He is an ASQ Fellow, Certified Quality Engineer, Quality Auditor, Quality Manager, Reliability Engineer, and Six Sigma Black Belt. He is also the author of numerous books on quality, productivity, and management.
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