ISO 39511:2018
Sequential sampling plans for inspection by variables for percent nonconforming (known standard deviation)
Sequential sampling plans for inspection by variables for percent nonconforming (known standard deviation)
- Статус документа:
- Действующий
- Формат:
- Электронный (PDF)
- Количество страниц:
- 41
- Дата публикации:
- 18 мая 2018 г.
- Издание:
- ISO IS 39511 edition 1 version 1
- ICS:
- 03.120.30
This International Standard specifies sequential sampling plans and procedures for inspection by variables of discrete items. The plans are indexed in terms of producer's risk point and the consumer's risk point. Therefore, they are suitable not only for the purposes of acceptance sampling, but for the more general purpose of the testing of simple statistical hypotheses for proportions. The purpose of this International Standard is to provide procedures for the sequential assessment of inspection results that may be used to induce the supplier to supply lots of a quality having a high probability of acceptance. At the same time, the consumer is protected by a prescribed upper limit to the probability of accepting a lot (or process) of poor quality. This International Standard is primarily designed for use under the following conditions: a) where the inspection procedure is to be applied to a continuing series of lots of discrete products all supplied by one producer using one production process. In such a case, sampling of particular lots is equivalent to the sampling of the process. If there are different producers or production processes, this International Standard shall be applied to each one separately; b) where only a single quality characteristic x of these products is taken into consideration, which must be measurable on a continuous scale; c) where the measurement error is negligible (i.e. with a standard deviation no more than 10 % of the process standard deviation); d) where production is stable (under statistical control) and the quality characteristic x has a known standard deviation, and is distributed according to a normal distribution or a close approximation to the normal distribution; CAUTION — The procedures in this International Standard are not suitable for application to lots that have been screened previously for nonconforming items. e) where a contract or standard defines an upper specification limit U, a lower specification limit L, or both; an item is qualified as conforming if and only if its measured quality characteristic, x, satisfies the appropriate one of the following inequalities: x ≤ U (i.e. the upper specification limit is not violated); x ≥ L (i.e. the lower specification limit is not violated); and (i.e. neither the upper nor the lower specification limit is violated.) Inequalities 1) and 2) are called cases with a "single specification limit", and 3) is the case with "double specification limits". In this International Standard, it is assumed that, where double specification limits apply, conformance to both specification limits is either equally important to the integrity of the product or is considered separately for both specification limits. In the first case, it is appropriate to control the combined percentage of product outside the two specification limits. This is referred to as combined control. In the second case, nonconformity beyond each of the limits is controlled separately, and this is referred to as separate control.
Abstract
Overview
ISO 39511:2018 - Sequential sampling plans for inspection by variables for percent nonconforming (known standard deviation) - specifies sequential sampling plans and procedures for inspecting discrete items when a single continuous quality characteristic is measured and the process standard deviation is known. The standard is indexed by producer’s risk point and consumer’s risk point, making it suitable for acceptance sampling and for testing simple statistical hypotheses about proportions. Its primary goal is to enable efficient, sequential assessment that encourages suppliers to maintain high-quality lots while protecting consumers against an upper bound on the probability of accepting poor-quality lots.
Key topics and technical requirements
- Scope conditions: applies to a continuing series of lots from one producer/process; a single measurable quality characteristic x; measurement error negligible (≤ 10% of process σ); known process standard deviation and approximately normal distribution.
- Risk indexing: plans are defined by producer’s risk point (PR) and consumer’s risk point (CR) to control operating characteristics.
- Sequential procedures: use cumulative statistics (leeway) to decide accept/reject decisions as sampling proceeds - reduces average sample size compared with single sampling plans.
- Curtailment: plans include pre‑set curtailment sample size (nt). If nt is reached without a decision, curtailment acceptance/rejection values are applied; curtailment is integral to the plan and affects operating characteristics.
- Methods provided: both numerical and graphical (acceptance charts) methods for single specification limits, combined control of double limits, and separate control of double limits.
- Limitations: not suitable for lots previously screened for nonconforming items; complexity and variable inspection effort can increase operational overhead.
- Design details: includes determination of acceptance and rejection values, curtailment values, and tables/illustrative examples (see Clause 7–9 and Annex A).
Practical applications and users
- Ideal where inspection of individual items is costly and reducing average sample size is desirable.
- Typical users: quality engineers, inspection managers, production statisticians, regulators, conformity assessment bodies, and suppliers/customers engaged in acceptance sampling or hypothesis testing for proportions.
- Common applications: process monitoring, supplier quality control in manufacturing, regulatory sampling where strict producer/consumer risk control is required, and situations demanding efficient inspection of very high-quality processes (parts‑per‑million nonconformance).
Related standards
- Supersedes ISO 8423:2008 (minor revision).
- Related to ISO 3951-1 and ISO 3951-5 (inspection by variables) - ISO 3951-5 provides sequential plans with a different design principle and switching rules for ongoing series; ISO 39511 is intended for strict risk control and hypothesis testing.
Keywords: ISO 39511:2018, sequential sampling plans, inspection by variables, percent nonconforming, known standard deviation, producer’s risk, consumer’s risk, curtailment, acceptance chart.
Технические детали
- Технический комитет
- ISO/TC 69/SC 5 - Acceptance sampling
- SKU
- ISO 39511:2018
Похожие стандарты
Стандарты, упомянутые в описании
ISO 8423:2008
ОтменёнSequential sampling plans for inspection by variables for percent nonconforming (known standard deviation)
BS ISO 39511:2018
ДействующийSequential sampling plans for inspection by variables for percent nonconforming (known standard deviation).
ISO 3951-5:2006
ДействующийSampling procedures for inspection by variables — Part 5: Sequential sampling plans indexed by acceptance qua…
Overview - ISO 3951-5:2006 (Sequential sampling plans, AQL, known σ) ISO 3951-5:2006 specifies a system of sequential sampling plans indexed by Acceptance Quality Limit (AQL) for inspection by variab…