Use Sequential Sampling to Reduce Attribute Sampling Costs

MessageThis Webinar is over
Date Feb 28, 2018
Time Wednesday, February 28, 2018 EST 13:00 Duration : 60 Minutes
Cost LIVE - SINGLE : $ 179.00
Description :
ANSI/ASQ Z1.9 (formerly MIL-STD 105) is an internationally recognized set of sampling plans for attribute (pass/fail, good/bad, or defect count) data. The standard specifies, on the basis of (1) the lot size N, (2) the inspection level, and (3) the acceptable quality level (AQL), a sample size n and acceptance number c. If the number of nonconformances exceeds the acceptance number, the lot is rejected. This does not mean the entire lot is thrown away, but rather that 100% inspection is required to remove all nonconforming items and possibly replacing them with known good ones. 

Inspection is, while a necessary or required (value-assisting activity), a non-value-adding one that does not actually create saleable product. This is why it is desirable to perform as little inspection as possible while continuing to meet customer requirements for quality assurance. ANSI/ASQ Z1.9 therefore offers double and multiple sampling plans that reject extremely bad lots very quickly and accept extremely good ones quickly. This reduces, on average, the total inspection performed. 

A sequential sampling plan, in which the successive sample size is 1, is the ultimate extension of a multiple sampling plan. It is relatively easy to define one on the basis of the corresponding ANSI/ASQ Z1.9 plan's AQL and producer's risk ?, and de facto rejectable quality level (RQL) and consumer's risk. (ANSI/ASQ Z1.9 does not have formal RQLs but the procedure pretends that the nonconforming fraction p for which the acceptance probability, or consumer's risk, is 10%, is the RQL.) The operating characteristic (OC) curve of the sequential plan will then match that of the ANSI/ASQ Z1.9 plan at (AQL, 1-?) and (RQL, ?)—that is, the chance of accepting a lot at the AQL will be 1-? (generally close to 95%) and accepting it at the RQL will be ? (10%). 

Sequential plans minimize the average sample number (ASN), but their standard form—in which each increment has its own acceptance and rejection number, and which can therefore consist of 100 or more tabular rows—is not particularly convenient for everyday use. Neither is the graphical version that requires the inspector to plot a point for each item inspected. This webinar offers a greatly reduced tabular format, often of 10 or fewer rows, in which the acceptance or rejection decision is based on the number of nonconformances found versus the number of items inspected.
Objective :
  • Use ANSI/ASQ Z1.9 to define a sampling plan on the basis of (1) the lot size N, (2) the inspection level, and (3) the acceptable quality level (AQL).
    • Be aware of the "chutes and ladders" (up and down arrows) aspect of the sampling tables.
    • Use ANSI/ASQ Z1.9's switching rules to move between normal, tightened, and reduced inspection based on quality performance and consistency of production.
  • Know how double and multiple sampling plans reduce the amount of inspection and therefore inspection costs
  • Generate a sequential sampling plan whose performance is identical to that of the corresponding ANSI/ASQ Z1.9 plan at the operating characteristic curve points (AQL, 1-a) and (RQL, b).
    • This can be done in Microsoft Excel, and attendees will be provided with a spreadsheet, or an off-the-shelf program like StatGraphics.
    • Tabulate the plan into a convenient and inspector-friendly format that assigns one row for the total number of nonconformances or defects found (0, 1, 2, …) and a corresponding acceptance and rejection sample size. As an example, we can accept the lot after inspection of the 40th item if no nonconformances have been found while, if we have found 2 nonconformances on or prior to the inspection of the 16th item, we must reject the lot.
    • Generate the operating characteristic curve to show that the sequential plan's performance is essentially identical to that of the ANSI/ASQ Z1.9 plan.
    • Compare the sequential plan's average sample number (ASN) to those of of the ANSI/ASQ Z1.9 single and double sampling plans.

Areas Covered in the Session :
  • Comprehensive overview of ANSI/ASQ Z1.9 including
    • Definition of the sample plan on the basis of the lot size, sampling level, and acceptable quality level
    • Invocation of normal, tightened, and reduced sampling on the basis of performance and consistency of production, according to the plan's switching rules.
    • Benefits of double and multiple sampling plans
  • Generation of a sequential sampling plan whose performance (operating characteristic curve) is essentially identical to that of the corresponding ANSI/ASQ Z1.9 plan
    • Tabulation of the sequential plan into a convenient and inspector-friendly format for easy use on the shop floor
    • Evaluation of the plan in terms of average sample number (ASN) and OC curve<

Who Will Benefit:
ALL Quality managers, engineers, and technicians with responsibility for lot inspections that use attribute data.
About Speaker:
William A. Levinson, P.E., FASQ, CFPIM, is the owner of Levinson Productivity Systems PC. He holds professional certifications from the American Society for Quality, APICS, and Society of Manufacturing Engineers.


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