TS ISO 3951-6
Sampling procedures for inspection by variables - Part 6: Specification for single sampling plans for isolated lot inspection indexed by limiting quality (LQ)
This document specifies an acceptance sampling system of single sampling plans for inspection by variables, primarily designed for use under the following conditions:
a) where the inspection procedure is applied to an isolated lot of discrete products all supplied by one producer using one production process;
b) where only a single quality characteristic, x, of this process is taken into consideration, which is measurable on a continuous scale;
c) where the quality characteristic, x, is distributed according to a normal distribution or a close approximation to a normal distribution;
d) where the quality characteristic can be measured without error or with moderate measurement error;
e) where a contract or standard defines a lower specification limit, L, an upper specification limit, U, 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:
1) x ≥ L (i.e. the lower specification limit is not violated);
2) x ≤ U (i.e. the upper specification limit is not violated);
3) x ≥ L and x ≤ U (i.e. neither the lower nor the upper specification limit is violated).
Inequalities 1) and 2) are cases with a single specification limit, whereas inequality 3) is a case with double specification limits.
Where double specification limits apply, it is assumed in this document that conformance to both specification limits is equally important to the integrity of the product. In such cases, it is appropriate to apply a single LQ to the combined fraction of a product outside the two specification limits. This is referred to as combined control.
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About This Item
TS ISO 3951-6 defines single sampling plans for inspection by variables using limiting quality (LQ) as the indexing basis. It addresses acceptance sampling for an isolated lot of discrete products from one producer and one production process, where the decision is made from measured values of a continuous quality characteristic rather than simple conforming/nonconforming counts.
In general technical terms, inspection by variables uses the actual size of a characteristic to estimate lot quality, so it can be more informative than attribute inspection when the measurement system is suitable. This standard is aimed at cases where the characteristic is normally distributed, or close to normal, and where measurement error is absent or moderate. It also assumes that conformity is defined by one lower specification limit, one upper specification limit, or both.
How the specification limits are used
For a single lower limit, an item conforms when x ≥ L; for a single upper limit, when x ≤ U; and for double specification limits, only when both conditions are met. The standard therefore frames conformity around the declared specification boundaries, not around a general notion of average quality. In the double-limit case, both limits are treated as equally important to product integrity, and a combined LQ is applied to the fraction of product outside either limit.
Why variable-based sampling is distinct
Variable sampling plans are widely used in manufacturing and supply situations where a measurable characteristic such as dimension, strength, mass, or concentration can be recorded on a continuous scale. Conceptually, this allows acceptance decisions to reflect not just whether items pass or fail, but how far the measured results sit from the specification limits. As a result, single sampling plans of this kind are a statistical tool for lot-by-lot acceptance decisions under clearly defined measurement and specification conditions.
- Publication Date: 4.03.2024
- Standard Status: U (Yürürlükteki Standard/Standard)
- Publisher: TSE
- Document Language: en
- ICS Classification: 03.120.30 İstatistiksel Metotların Uygulanması
- Pages: 91
- Source Standard: ISO 3951-6:2023
- International Equivalents: ISO 3951-6-EQV
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