TS ISO 3951-4
Sampling procedures for inspection by variables - Part 4: Procedures for assessment of declared quality levels
This part of ISO 3951 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 (see Clause 4). 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 this part of ISO 3951 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.
This part of ISO 3951 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 in this part of ISO 3951 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, and
? 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.
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About This Item
TS ISO 3951-4 addresses sampling by variables for assessing whether a declared quality level is supported by sample evidence. In practical terms, it is a statistical inspection standard for judging a stated level of quality from measured data, rather than a lot-acceptance standard in the usual sense.
How this part uses measured data to judge a declared quality level
The standard provides sampling plans for measurable quality characteristics that are assumed to be independent and normally distributed. In general technical terms, inspection by variables uses the actual numerical values of measurements, which can give more sensitive information than simple conforming/nonconforming classification. That makes it suitable when the interest is the fraction nonconforming and when the distributional behavior of the characteristic can be modelled.
This part includes plans for unknown and known process standard deviation, and for three levels of discriminatory ability. It is intended for entities such as lots, process output and similar sources from which random samples of individual items can be taken. Unlike the other parts of ISO 3951, its procedures are not applicable to acceptance assessment of lots; instead, they are aimed at objective evidence for or against a declared quality level. The plans are designed so their operating characteristic curves closely match the corresponding attributes plans in ISO 2859-4, helping keep the choice of inspection method from being driven by the chance of accepting an incorrectly declared quality level.
- Typical application areas include end items, components and raw materials.
- They also extend to operations, work-in-process material and supplies in storage.
- Maintenance operations, data or records, and administrative procedures are included as possible subjects of inspection.
- The procedure balances the risk of contradicting a correct declaration and the risk of failing to contradict an incorrect one.
- Publication Date: 15.02.2021
- Standard Status: U (Yürürlükteki Standard/Standard)
- Publisher: TSE
- Document Language: en
- ICS Classification: 03.120.30 İstatistiksel Metotların Uygulanması
- Pages: 35
- Source Standard: ISO 3951-4:2011
- International Equivalents: ISO 3951-4-EQV; BS ISO 3951-4-EQV
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