National foreword

This British Standard is the UK implementation of ISO 2859‑4:2020.

The UK participation in its preparation was entrusted to Technical Committee SS/5, Acceptance sampling schemes.

A list of organizations represented on this committee can be obtained on request to its secretary.

This publication does not purport to include all the necessary provisions of a contract. Users are responsible for its correct application.

Compliance with a British Standard cannot confer immunity from legal obligations.

This British Standard was published under the authority of the Standards Policy and Strategy Committee on 30 June 2020.

Foreword

ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies (ISO member bodies). The work of preparing International Standards is normally carried out through ISO technical committees. Each member body interested in a subject for which a technical committee has been established has the right to be represented on that committee. International organizations, governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.

The procedures used to develop this document and those intended for its further maintenance are described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types of ISO documents should be noted. This document was drafted in accordance with the editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).

Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of any patent rights identified during the development of the document will be in the Introduction and/or on the ISO list of patent declarations received (see www.iso.org/patents).

Any trade name used in this document is information given for the convenience of users and does not constitute an endorsement.

For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions related to conformity assessment, as well as information about ISO's adherence to the World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.

This document was prepared by Technical Committee ISO/TC 69, Application of statistical methods, Subcommittee SC 5, Acceptance sampling.

This third edition cancels and replaces the second edition (ISO 2859‑4:2002), which has been technically revised.

The main changes compared to the previous edition are as follows:

the scope has been widened from testing for the percentage of nonconforming items to testing for the percentage of nonconformities per item;
the sampling and decision algorithm is more detailed and has been moved to an earlier position in the document, so as to facilitate the operational use of the document;
an LQR level 0 has been introduced, which can be used when a large probability of erroneously contradicting a correctly declared quality level can be tolerated;
an informative Annex B has been added to explain the mathematical-statistical background, and to provide tables on quality ratios.

A list of all parts in the ISO 2859 series can be found on the ISO website.

Any feedback or questions on this document should be directed to the user’s national standards body. A complete listing of these bodies can be found at www.iso.org/members.html.

Introduction

The procedures in this document differ in their scope from the procedures in ISO 2859‑1 to ISO 2859‑3. The acceptance sampling systems specified in ISO 2859‑1 to ISO 2859‑3 are intended to be used in bilateral agreements between two parties. The acceptance sampling procedures are supposed to be used as simple, pragmatic rules for deciding on product release by inspection of only a limited sample of a consignment, and therefore the procedures do not make reference (either explicitly or implicitly) to any formally declared quality level.

Under acceptance sampling there is no sharp borderline between quality levels that should be considered acceptable and qualities that should be rejected by the procedure. For the procedures in ISO 2859‑1, the two parties agree upon some acceptance quality limit (AQL) which is the worst tolerable process average when a continuing series of lots is submitted. The switching rules and the sampling schemes in ISO 2859‑1 are designed to encourage the suppliers to have process averages consistently better than the AQL selected. In order to keep sample sizes moderate, the protection against accepting individual lots of inferior quality may be less than that provided by sampling plans targeted for sentencing individual lots. The procedures in ISO 2859‑2, on the contrary, are designed to provide good protection against accepting individual lots of inferior quality (LQ), but at the expense of a possibly high risk of not accepting lots of qualities that both parties actually would consider to be acceptable.

The procedures in ISO 2859‑1 to ISO 2859‑3 are well suited for acceptance sampling purposes, but they should not be used in reviews, audits, systematic tests, etc. to verify a quality that has been declared for some entity. The main reason is that the procedures have been indexed in terms of quality levels that are relevant solely for the pragmatic purposes of acceptance sampling, and the various risks have been balanced accordingly in a pragmatic attitude.

The procedures in this document have been developed as a response to the growing need for sampling procedures suitable for formal, systematic inspections such as reviews or audits or systematic tests. When performing such a formal inspection, it is necessary both for the inspecting authority and for the body subject to inspection to consider the risks of reaching an incorrect conclusion. These risks have to be accounted for explicitly in the design of review/auditing/testing procedures.

This document provides guidance and rules to assist the user in accounting for the risks of incorrect conclusions in an informed manner.

The rules in this document have been devised such that there is only an acceptably small risk of contradicting the declared quality level when in fact the actual level conforms to the declared level.

If it were also desired that there should be a similarly small risk of not contradicting the declared quality level when in fact the actual quality level does not conform to the declared quality level, then it would be necessary to investigate a rather large sample. Therefore, in order to obtain the benefit of a moderate sample size, the procedures in this document have been devised in such a way that they allow a somewhat higher risk of failing to contradict the declared quality level when in fact the actual quality level does not conform to the declared quality level.

The wording of the result of the assessment should reflect this imbalance between the risks of reaching incorrect conclusions. For the levels I, II, and III, when the sample result contradicts the declared quality level, there is strong evidence of nonconformance to the declared quality level. When the sample result does not contradict the declared quality level, this should be understood as “we have not, in this limited sample, found strong evidence of nonconformance to the declared quality level”.

CAUTION — It should be noticed that, for sampling plans with very small sample sizes, one should be aware of the poor discriminatory power under such sample sizes by referring to the entries in Tables B.1, B.2, B.3 and B.4.

1 Scope

This document establishes single sampling plans for conformance testing, i.e., for assessing whether the quality level of a relevant audit population (lot, process, inventory, file etc) conforms to a declared value. Sampling plans are provided corresponding to four levels of discriminatory ability. The limiting quality ratio (LQR) (see Clause 4) of each sampling plan is given for reference. For levels I-III, the sampling plans have been devised so as to obtain a risk no more than 5 % of contradicting a correct declared quality level. The risk of failing to contradict an incorrectly declared quality level which is related to the LQR is no more than 10 %. The sample sizes for level 0 are designed in a way that the LQR factors of the sampling plans are compatible with the LQR factors for level I.

In contrast to the procedures in the other parts of the ISO 2859 series, the procedures in this document are not applicable to acceptance assessment of lots. Generally, this document mainly focuses on controlling type I error, which differs from the balancing of the risks in the procedures for acceptance sampling.

This document can 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 document are applicable, but not limited, to the inspection of a variety of targets such as:

end items;
components and raw materials;
operations;
materials in process;
supplies in storage;
maintenance operations;
data or records;
administrative procedures;
accounting procedures or accounting entries;
internal control procedures.

This document considers two types of quality models for discrete items and populations, as follows.

The conforming-nonconforming model, where each item is classified as conforming or nonconforming, and where the quality indicator of a population of items is the proportion p of nonconforming items, or, equivalently, the percentage 100 p of nonconforming items.
The nonconformities model, where the number of nonconformities is counted on each item, and where the quality indicator of a population of items is the average number λ of nonconformities found on items in the population, or, equivalently, the percentage 100 λ of nonconformities on items in the population.

2 Normative references

3 Terms, definitions, symbols and abbreviated terms

3.1 Terms and definitions

3.2 Symbols and abbreviated terms

4 Sampling and decision procedure

4.1 Identifying a sampling plan

4.2 Drawing of samples

4.3 Decision objective

4.4 Decision by sampling

4.5 Disposition of nonconforming items

5 Principles

6 Declared quality level (DQL)

7 Sampling plans

7.1 General

7.2 LQR level 0

7.3 LQR level I

7.4 LQR level II

7.5 LQR level III

8 Further information

8.1 Curves showing the approximate probability of contradiction

8.2 Tables indicating discriminatory ability

BS ISO 2859-4:2020

Sampling procedures for inspection by attributes - Procedures for assessment of declared quality levels

Current, Under Review

Published:

30 Jun 2020