Reduce Inspection Costs with Sequential Sampling

Why Should You Attend:

ANSI/ASQ Z1.4 is a basic and widely used sampling plan in many industries. It is therefore important to understand not only its mechanics but also switching rules that depend on process performance. The switching rules allow reduced inspection when quality is very good, but require tightened inspection when there is evidence that process performance has deteriorated.

Inspection does not, however, add actual value to the product. It is therefore desirable to minimize it as long as the inspection plan still meets quality requirements. This is where double, multiple, and sequential sampling plans come into play, and this webinar will show how to define a sequential sampling plan in a simple and inspector-friendly format.

Areas Covered in the Webinar:

• How to use ANSI/ASQ Z1.4 to define sampling plans based on (1) lot size N, (2) the acceptable quality level (AQL), and (3) the specified inspection level.
• ANSI/ASQ Z1.4's switching rules that allow transition to reduced inspection when quality is good, and require tightened inspection when quality problems occur.
• An inspection plan format that reduces the plan to ten or fewer rows in most cases.
• How to calculate the ASN as well as the probability of acceptance at any given quality level (nonconforming fraction); the latter defines the operating characteristic (OC) curve.

Who Will Benefit:

• Quality Managers
• Manufacturing Professionals
• Audit Managers
• Quality Auditors
• Regulatory Affairs Managers

William Levinson
Principal Consultant, Levinson Productivity Systems

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 several books on quality, productivity, and management.

Topic Background:

ANSI/ASQ Z1.4 (formerly Mil-STD 105) is a standard for quality sampling by attributes, and it is frequently used for acceptance sampling in incoming, in-process, and outgoing inspections. It defines a sample plan in terms of sample size n and acceptance number c based on the sampling level, acceptable quality level (AQL) and lot size. It is however possible to reduce the sample size, and inspection costs, with sequential sampling plans.

Sampling by attributes, in which a sample is inspected to determine whether a production lot should be accepted or rejected, is a basic quality assurance technique. The simplest plan involves inspection of n pieces and, if more than the acceptance number (c) are nonconforming, then the lot is rejected. This does not usually mean the lot is thrown away; it is subjected to 100 percent inspection to sort out all the nonconforming units. Rejection may also invoke switching rules that require more stringent inspection of subsequent lots.

Inspection is however a mandatory but non-value-adding activity, which means we should do as little as possible consistent with quality requirements as expressed in terms of acceptable quality level (AQL) and specified inspection level. ANSI/ASQ Z1.4 includes double and multiple sampling plans whose purpose is to reject very bad lots quickly, and accept very good ones quickly. This usually results in less inspection.

The ultimate extension of the multiple sampling plan is the sequential sampling plan, in which units are inspected one at a time until a decision point is reached. This minimizes the average sample number, and therefore the inspection cost.

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