1、 Reference number ISO/TR 8550-1:2007(E) ISO 2007TECHNICAL REPORT ISO/TR 8550-1 First edition 2007-06-01 Guidance on the selection and usage of acceptance sampling systems for inspection of discrete items in lots Part 1: Acceptance sampling Lignes directrices pour la slection dun systme, dun programm
2、e ou dun plan dchantillonnage pour acceptation pour le contrle dunits discrtes en lots Partie 1: Lignes directrices gnrales pour lchantillonnage pour acceptation ISO/TR 8550-1:2007(E) PDF disclaimer This PDF file may contain embedded typefaces. In accordance with Adobes licensing policy, this file m
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7、 22 749 01 11 Fax + 41 22 749 09 47 E-mail copyrightiso.org Web www.iso.org Published in Switzerland ii ISO 2007 All rights reservedISO/TR 8550-1:2007(E) ISO 2007 All rights reserved iiiContents Page Foreword iv Introduction v 1 Scope . 1 2 Normative references . 1 3 Terms and definitions. 2 4 Abuse
8、s and uses of acceptance sampling 2 5 Acceptance sampling plans, schemes and systems 5 6 Practical and economic advantages of using standard sampling plans 5 7 Attributes versus variables 7 8 Further considerations influencing a selection. 8 9 Making a comparison of the methods for sampling inspecti
9、on 23 10 Other methods sometimes adopted in practice 29 11 Relevance of market and production conditions 31 12 The final selection Realism . 32 Annex A (informative) Example of a simple model for profit maximization under destructive inspection by attributes . 33 Bibliography . 37 ISO/TR 8550-1:2007
10、(E) iv ISO 2007 All rights reservedForeword 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 in
11、terested 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 Electrotechni
12、cal Commission (IEC) on all matters of electrotechnical standardization. International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2. The main task of technical committees is to prepare International Standards. Draft International Standards adopted by the
13、 technical committees are circulated to the member bodies for voting. Publication as an International Standard requires approval by at least 75 % of the member bodies casting a vote. In exceptional circumstances, when a technical committee has collected data of a different kind from that which is no
14、rmally published as an International Standard (“state of the art”, for example), it may decide by a simple majority vote of its participating members to publish a Technical Report. A Technical Report is entirely informative in nature and does not have to be reviewed until the data it provides are co
15、nsidered to be no longer valid or useful. 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. ISO/TR 8550-1 was prepared by Technical Committee ISO/TC 69
16、, Applications of statistical methods, Subcommittee SC 5, Acceptance sampling. This first edition of ISO/TR 8550-1, together with ISO/TR 8550-2 and ISO/TR 8550-3, cancels and replaces ISO/TR 8550:1994. ISO/TR 8550 consists of the following parts, under the general title Guidance on the selection and
17、 usage of acceptance sampling systems for inspection of discrete items in lots: Part 1: Acceptance sampling Part 3: Sampling by variables The following part is under preparation: Part 2: Sampling by attributes ISO/TR 8550-1:2007(E) ISO 2007 All rights reserved vIntroduction This part of ISO/TR 8550
18、gives guidance on the selection of an appropriate acceptance sampling scheme for the inspection of discrete items submitted in lots from the schemes described in various national and international standards. There are many situations where products (materials, parts, components, assemblies and syste
19、ms) are transferred from one organization to another, where the organizations may be different companies or parts of a single company or even different shops within a plant. In these situations both the supplier and the customer may use acceptance sampling procedures to satisfy themselves that the p
20、roduct is of acceptable quality. Suppliers will be seeking to maintain a reputation for good quality and to reduce the likelihood of claims under warranty, but without incurring unnecessary production and supply costs. On the other hand, customers will require adequate evidence, at minimum cost to t
21、hemselves, that the product they receive conforms to specifications. Compared with, say, 100 % inspection, suitable sampling methods will often be beneficial in achieving these aims. Sometimes acceptance sampling methods are the only practical procedure, especially when the tests for conformance are
22、 destructive. Several types of sampling systems, schemes and plans are available for these purposes. They are presented in a number of ISO Standards that explain how they are to be used. However, it is often difficult to decide on the most appropriate procedure for use in a particular situation. The
23、 purpose of this part of ISO/TR 8550 is to assist in that decision. The choice of sampling system, scheme or plan depends on a number of conditions and on the prevailing circumstances. In any supply situation, the first essential is that the supplier and the customer understand, and have agreed upon
24、, the requirements and the basis for release and acceptance of the product, including any acceptance sampling methods to be used. Lots that are non-acceptable cause difficulties for both supplier and customer. The supplier incurs additional costs in rework, scrap, increased inspection, damage to rep
25、utation and possibly loss of sales. Delays in delivery and re-inspection costs are a burden to the customer. For these reasons, it is usually considered essential for the supplier to provide lots that have a very high probability of being accepted, i.e. 95 % or more. The supplier has to ensure that
26、quality control of the production or delivery process provides lots of a quality sufficient to meet this objective. A basic principle of some acceptance sampling inspection schemes is to promote the production of lots of acceptable quality. The primary purpose of these schemes is not to discriminate
27、 between acceptable and non-acceptable lots, i.e. to sort, but to keep production under control to yield an acceptable process average quality. Although all acceptance sampling plans are discriminatory to some degree, the process average quality (expressed in terms of percent nonconforming or number
28、 of nonconformities) should not be greater than half the acceptance quality limit in order to ensure a very high probability of acceptance. The primary purpose of the ISO/TR 8550 series is to give guidance on the selection of an acceptance sampling system, scheme or plan. It does this principally by
29、 reviewing the available systems specified by various standards and showing ways in which these can be compared in order to assess their suitability for an intended application. The guide also indicates how prior knowledge of the manufacturing or service delivery process and quality performance migh
30、t influence the choice of sampling system, scheme or plan, and likewise how the particular needs of the customer affect selection. Some specific circumstances encountered in practice are described and the method of choosing a plan is explained. Some checklists or pointers and tables are provided to
31、assist users in selecting an appropriate system, scheme or plan for their purposes. Charts are included to illustrate the procedures to be followed in the selection process. This publication does not purport to include all the necessary provisions of a contract. Users are responsible for its correct
32、 application. TECHNICAL REPORT ISO/TR 8550-1:2007(E) ISO 2007 All rights reserved 1Guidance on the selection and usage of acceptance sampling systems for inspection of discrete items in lots Part 1: Acceptance sampling 1 Scope This part of ISO/TR 8550 gives general guidance on the selection of an ac
33、ceptance sampling system, scheme or plan. It does this principally in the context of standards that either already exist or are presently under development. (For more detailed information about specific acceptance sampling systems, see ISO/TR 8550-2 for sampling by attributes or ISO/TR 8550-3 for sa
34、mpling by variables.) The guidance in this part of ISO/TR 8550 is confined to acceptance sampling of products that are supplied in lots and that can be classified as consisting of discrete items (i.e. discrete articles of product). It is assumed that each item in a lot can be identified and segregat
35、ed from the other items in the lot and has an equal chance of being included in the sample. Each item of product is countable and has specific characteristics that are measurable or classifiable as being conforming or nonconforming (to a given product specification). Standards on acceptance sampling
36、 are typically generic, as a result of which they can be applied to a wide variety of inspection situations. These include, but are not limited to, the following: a) end items, such as complete products or sub-assemblies; b) components and raw materials; c) services; d) materials in process; e) supp
37、lies in storage; f) maintenance operations; g) data or records; h) administrative procedures. Although this part of ISO/TR 8550 is written principally in terms of manufacture and production, this should be interpreted liberally, as it is applicable to the selection of sampling systems, schemes and p
38、lans for all types of products and processes as defined in ISO 9000. 2 Normative references The following referenced documents are indispensable for the application of this document. For dated references, only the edition listed applies. For undated references, the latest edition of the referenced d
39、ocument (including any amendment) applies. ISO 3534-1, Statistics Vocabulary and symbols Part 1: General statistical terms and terms used in probability ISO/TR 8550-1:2007(E) 2 ISO 2007 All rights reservedISO 3534-2, Statistics Vocabulary and symbols Part 2: Applied statistics ISO 9000, Quality mana
40、gement systems Fundamentals and vocabulary 3 Terms and definitions For the purposes of this part of ISO/TR 8550, the terms and definitions given in ISO 3534-1, ISO 3534-2 and ISO 9000 apply. 4 Abuses and uses of acceptance sampling 4.1 Abuses of acceptance sampling Acceptance sampling has become unp
41、opular since the early 1980s. Some of the reasons for this (although certainly not all) are well founded, so it is important to be able to distinguish those situations where acceptance sampling should not be used from those where it may be appropriate. The chief arguments used against the use of acc
42、eptance sampling are as follows. a) When quality is generally very high, the sample sizes needed to detect a slip in quality are uneconomically large. b) Quality cannot be inspected into a product. c) It is far better to establish a robust design and to implement comprehensive process controls than
43、to try to find and eliminate nonconforming items after manufacture. d) Most acceptance sampling standards are indexed in terms of acceptable quality level (AQL). Once an AQL has been established and quality has been brought sufficiently below the AQL to achieve high probabilities of lot acceptance,
44、there is no incentive for the producer to try continuously to improve quality. e) Quoting an AQL is tantamount to granting a licence to produce defects. f) The only acceptable quality level is zero defects. These arguments are examined in turn in the following subclauses. 4.2 Example 1 The following
45、 simplified example, devised by Baillie 18 , demonstrates how the optimum sampling plan can vary according to the quality level against which it is desired to guard. A certain item is produced in lots of size 10 000, with a unit production cost of 10,00. The selling price per item is a in accepted l
46、ots and at a discounted price of 0,50 in lots non-accepted by the acceptance procedure. Testing is destructive, and the cost of testing each item is 1,00. The downstream cost (e.g. warranty cost plus loss of goodwill) of a nonconforming item in an accepted lot is 10 000, but zero in non-accepted lot
47、s sold at a discount. Historical data indicate that the process fraction nonconforming is p for 99 % of lots, but that it unaccountably and randomly slips to 100p for 1 % of the lots. A single sampling plan by attributes is to be used, i.e. a random sample of size n is to be selected from each lot,
48、and the lot is to be considered acceptable if the sample contains no more than Ac nonconforming items. What is the optimal sampling plan, i.e. the plan that maximizes the profit per item sold? Mathematical details are provided in Annex A for information. Table 1 shows the optimal sampling plan for a
49、 range of values of the process quality level p. The results are instructive. ISO/TR 8550-1:2007(E) ISO 2007 All rights reserved 3Table 1 Optimal sampling plans for Example 1 Optimal plan Usual quality level, in fraction nonconforming Quality level after slippage, in fraction nonconforming Sample size n Acceptance number, Ac Selling price per item, a () Average profit per item sold () 0,001 00 0,100 104 2 20,25 0,022 0,000 50 0,050 139 1 15,40 0,091 0,000 30 0,030 197 1 13,60 0,211 0,000 20 0,020 249 1 12,75
