ISO 3951-4:2011
Sampling procedures for inspection by variables — Part 4: Procedures for assessment of declared quality levels
Sampling procedures for inspection by variables — Part 4: Procedures for assessment of declared quality levels
- Статус документа:
- Действующий
- Формат:
- Электронный (PDF)
- Количество страниц:
- 25
- Дата публикации:
- 4 августа 2011 г.
- Издание:
- ISO IS 3951 edition 1 version 1
- ICS:
- 03.120.30
ISO 3951-4:2011 establishes sampling plans and procedures by variables that can be used to assess whether the quality level of an entity (lot, process, etc.) conforms to a declared value. The sampling plans have been devised so that their operating characteristic curves match those of the corresponding attributes plans in ISO 2859-4 as closely as possible, so that the choice between using sampling by attributes and sampling by variables is not influenced by attempts to increase the chance of accepting an incorrectly declared quality level. In this part of ISO 3951, there is a risk of between 1,4 % and 8,2 % of contradicting a correct declared quality level. The risk is 10 % of failing to contradict an incorrect declared quality level which is related to the limiting quality ratio. Sampling plans are provided corresponding to three levels of discriminatory ability, and for the cases of unknown and known process standard deviation. In contrast to the procedures in the other parts of ISO 3951, the procedures in ISO 3951-4:2011 are not applicable to acceptance assessment of lots. Generally, the balancing of the risks of reaching incorrect conclusions in assessment procedures will differ from the balancing in the procedures for acceptance sampling. ISO 3951-4:2011 may be used for various forms of quality inspection in situations where objective evidence of conformity to some declared quality level is to be provided by means of inspection of a sample. The procedures are applicable to entities such as lots, process output, etc. that allow random samples of individual items to be taken from the entity. The sampling plans provided are applicable, but not limited, to inspection of a variety of products such as: end items; components and raw materials; operations; materials in process; supplies in storage; maintenance operations; data or records; administrative procedures. The procedures are intended to be used when the quality characteristics are measurable variables that are independent and normally distributed, and where the quantity of interest is the fraction of items that are nonconforming.
Abstract
Overview
ISO 3951-4:2011 - "Sampling procedures for inspection by variables - Part 4: Procedures for assessment of declared quality levels" specifies variable-based sampling plans and procedures to assess whether an entity’s quality level conforms to a declared value (DQL). The standard provides sampling plans for measurable quality characteristics assumed independent and normally distributed, and targets the fraction nonconforming as the quantity of interest. Plans are matched as closely as possible to the operating characteristic (OC) curves of the corresponding attributes plans in ISO 2859-4 to avoid bias when choosing between attributes and variables sampling.
Key risk parameters are defined: there is roughly a 1.4% to 8.2% risk of contradicting a correct declared quality level and a 10% risk of failing to contradict an incorrect declared quality level at the Limiting Quality Ratio (LQR). Sampling options are provided for three LQR discrimination levels and for both unknown and known process standard deviation (the “s” and “σ” methods).
Key Topics
- Declared Quality Level (DQL): The benchmark quality level used to index sampling plans; must be justifiable and not estimated from the sample under inspection.
- Limiting Quality Ratio (LQR): Ratio of actual to declared quality used to control the risk of failing to contradict an incorrect DQL; three LQR levels are provided.
- Sampling plans and OC matching: Plans are selected so that variable-sampling OCs approximate ISO 2859-4 attribute OCs (Annex A describes the matching method).
- “s” and “σ” methods: Sampling procedures provided for unknown (sample standard deviation) and known (process standard deviation) variability.
- Risk balance: The standard intentionally balances a low risk of contradicting a correct DQL against a higher risk of failing to contradict an incorrect DQL to keep sample sizes moderate.
- Scope limits: Not intended for lot acceptance sampling; applicable to assessments, audits and formal inspections.
Applications
ISO 3951-4 is suited for objective, formal assessments where evidence is required that a process, lot or other entity matches a declared quality level. Typical uses:
- Quality audits, regulatory reviews, supplier assessments and certification sampling
- Process output verification, in‑process inspection and maintenance operation checks
- Inspection of end items, components, raw materials, records or administrative procedures
- Situations requiring statistical evidence of conformance based on measurable variables
Who uses this standard
- Quality managers and auditors
- Regulatory bodies and certification authorities
- Industrial statisticians and process engineers
- Supplier quality and compliance teams
Related Standards
- ISO 2859-4:2002 - Sampling procedures for inspection by attributes - Part 4: Procedures for assessment of declared quality levels
- ISO 3951-2:2006 - Sampling procedures for inspection by variables - General specification for single sampling plans (AQL)
- ISO 3951 (other parts) - single, double and sequential plans by variables
- ISO 3534-1/2 and ISO 9000 - statistical and quality vocabulary
Keywords: ISO 3951-4, sampling by variables, declared quality level (DQL), limiting quality ratio (LQR), sampling plans, operating characteristic, quality inspection, “s” method, “σ” method.
Технические детали
- Технический комитет
- ISO/TC 69/SC 5 - Acceptance sampling
- SKU
- ISO 3951-4:2011
Похожие стандарты
Стандарты, упомянутые в описании