Learn How to Design Sequential Sampling Plans that Deliver Benefits
Traditional ANSI/ASQ Z1.4 sampling plans specify, for a given inspection level and acceptable quality level (AQL), a sample size n and acceptance number c. ANSI/ASQ Z1.4 also offers double and multiple sampling plans that reduce the average total inspection (ATI) by accepting very good lots on the first sample, and similarly rejecting very bad lots.
The limiting case of a multiple sampling plan is a sequential sampling plan in which each successive sample consists of a single unit. These plans are designed to accept very good lots as quickly as possible, and reject very bad ones up front. Their ATIs are accordingly lower than those of ANSI/ASQ Z1.4 single, double, or multiple sampling plans, and they can be shown to offer similar protection (per their operating characteristic curves) against poor quality.
In this webinar, our expert speaker William A. Levinson will show you how to design sequential sampling plans in ANSI/ASQ Z1.4 the reduce inspection costs and deliver other benefits. The presentation will include an Excel spreadsheet that can create sampling plans of this nature based on any ANSI/ASQ Z1.4 normal or tightened inspection plan.
- Inspection is a value-assisting but non-value-adding activity. That is, it may be necessary and, if we stopped doing it, we might soon notice its absence, but we can save money by doing as little as possible while we still provide the customer with the same level of protection from poor quality.
- Any ANSI/ASQ Z1.4 plan can be translated into a sequential sampling plan with a lower ATI. The plan can be presented as a visual control in which the inspector (or computer) crosses off successive rows as nonconformances are found, with the next row showing how many items (k or less) must have been inspected to reject the lot, and how many must have been inspected to pass the lot.
- The OC curve of the sequential plan can then be generated for comparison to that of the ANSI/ASQ Z1.4 plan to satisfy the customer that it provides similar protection against poor quality.
- An ANSI/ASQ Z1.4 sample plan is defined by a sample size n and acceptance number c. These in turn dictate the producer's risk (alpha) of rejecting a lot at the AQL, and this risk is generally targeted at 5%. While these plans are not based on a rejectable quality level (RQL), we can calculate the quality level at which the plan has a 90% chance of rejecting the lot and treating it as the RQL.
- A sequential sampling plan can then be developed based on the two points (AQL, alpha) and (RQL, beta) where beta, the consumer's risk of accepting a lot at the RQL, is traditionally 10 percent. The resulting plan has acceptance and rejection numbers for every successive item that is inspected. It is alternatively possible to state, in spreadsheet form, the number of items that must be inspected to pass the lot for a given number of nonconforming pieces, and also the number inspected vs. number of nonconformances found to reject the lot. This format is far more convenient than a table that might contain hundreds of rows.
- It is then possible to compute the operating characteristic (OC) curve, which plots the chance of acceptance versus the actual nonconforming fraction, and compare it to the OC curve for the corresponding ANSI/ASQ Z1.4 plan. The two will generally be very similar and, of course, the points (AQL, alpha) and (RQL, beta) will match by design.
- The average total inspection (ATI) can also be calculated versus the actual nonconforming fraction for comparison to that of an ANSI/ASQ Z1.4 plan.
- Sequential sampling plans can also be used for tightened ANSI/ASQ Z1.4 plans (per the switching rules) but the issue of reduced sampling plans is more complicated. However, less is to be gained from sequential sampling because the ANSI/ASQ Z1.4 reduced sampling plan involves less inspection by definition.
Who should attend?
- Quality managers
Ask a question at the Q&A session following the live event and get advice unique to your situation, directly from our expert speaker.
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