1、BRITISH STANDARD BS 6000-2:2005 Guide to the selection and usage of acceptance sampling systems for inspection of discrete items in lots Part 2: Guide to sampling by attributes ICS 03.120.30 BS 6000-2:2005 This British Standard was published under the authority of the Standards Policy and Strategy C
2、ommittee on 25 July 2005 BSI 25 July 2005 First published as BS 6000 March 1972 Second edition May 1996 BS 6000-2 first edition published 25 July 2005 The following BSI references relate to the work on this British Standard: Committee reference SS/5 Draft for Development 03/102302 DC ISBN 0 580 4648
3、5 7 Committees responsible for this British Standard The preparation of this British Standard was entrusted to Technical Committee SS/5, Acceptance sampling schemes, upon which the following bodies were represented: Association for Road Traffic and Safety Management British Measurement and Testing A
4、ssociation City University Clay Pipe Development Association Ltd. Institute of Metal Finishing Institute of Quality Assurance Ministry of Defence UK Defence Standardization Co-opted members Amendments issued since publication Amd. No. Date CommentsBS 6000-2:2005 BSI 25 July 2005 i Contents Page Comm
5、ittees responsible Inside front cover Foreword ii 1S c o p e 1 2 The selection process 1 3 British Standards for acceptance sampling of lots by attributes 2 4 Effect on the selection process of market and production conditions 5 Bibliography 13 Table 1 Guide for selection of a candidate acceptance s
6、ampling system, scheme or plan for inspection by attributes, based on the inspection situation 6 Table 2 Guide for the selection of an acceptance sampling system, scheme or plan for sampling by attributes, using existing market conditions 7 Table 3 Guide for the selection of an acceptance sampling s
7、ystem, scheme or plan for sampling by attributes, using existing production conditions 8 Figure 1 Illustration of the selection procedure for inspection by attributes when production is continuous and run length exceeds 10 lots on original inspection 10 Figure 2 Illustration of the selection procedu
8、re for inspection by attributes when production is not continuous or run length is 10 lots or fewer on original inspection 11BS 6000-2:2005 ii BSI 25 July 2005 Foreword This part of BS 6000 has been prepared by Technical Committee SS/5. Together with BS 6000-1 and BS 6000-3 it supersedes BS 6000:199
9、6, which is withdrawn. BS 6000:2005, Guide to the selection and usage of acceptance sampling systems for inspection of discrete items in lots is in three parts: Part 1: General guide to acceptance sampling Part 2: Guide to sampling by attributes Part 3: Guide to sampling by variables This publicatio
10、n does not purport to include all the necessary provisions of a contract. Users are responsible for its correct application. Compliance with a British Standard does not of itself confer immunity from legal obligations. Summary of pages This document comprises a front cover, an inside front cover, pa
11、ges i to ii, pages 1 to 13 and a back cover. The BSI copyright notice displayed in this document indicates when the document was last issued.BS 6000-2:2005 BSI 25 July 2005 1 1 Scope This part of BS 6000 gives guidance on the selection of an acceptance sampling system, scheme or plan for inspection
12、by attributes. It does this principally by reviewing the available systems specified by various British Standards and showing ways in which these can be compared in order to assess their suitability for an intended application. It is assumed that the choice has already been made to use sampling by a
13、ttributes in preference to sampling by variables. The guidance in BS 6000-2 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). Each item in a lot can be identified and segregated fro
14、m 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 specification). Standards on acceptance sampling by attributes
15、 are applicable 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) supplies in storage; f) maintenance operations; g) da
16、ta or records; and h) administrative procedures. Although this British Standard is written principally in terms of manufacture and production, it is applicable to the selection of sampling systems, schemes and plans for all types of product and processes as defined in BS EN ISO 9000. 2 The selection
17、 process The task of selecting a suitable sampling system, scheme or plan is influenced by production and marketing conditions. In addition, the economics of the sampling system, the resources of the inspection organization and other aspects need to be considered. Therefore, the selection process be
18、comes complex and rarely is there one method of acceptance sampling that fits all situations even though they might appear to be similar. Table 1, Table 2 and Table 3, together with Figure 1 and Figure 2, illustrate the process of selecting a sampling system, scheme or plan. The tables provide “cand
19、idate” sampling systems, schemes and plans to fit given inspection situations, production conditions and market conditions, respectively. The method suggested is to review Table 1, Table 2 and Table 3 and to select as many “candidate plans” as fit the situation. These candidates should then be revie
20、wed through Figure 1 or Figure 2 so that the user finally arrives at a system, scheme or plan that is most feasible and economical for the situation. The initial selection decisions are for long and short production runs, isolated lots, and attribute or variables sampling; these decisions will point
21、 to one or more possible sampling schemes specified in standards. Figure 1 and Figure 2 are not flow charts. They show, in summary form, the procedures contained within each standard for arriving at a sampling system, scheme or plan. The summaries are listed side by side, which allows direct compari
22、son between them. The solid connecting lines indicate the primary course to be followed in choosing and operating a standard; the broken lines indicate alternatives. These alternatives are applicable only under certain conditions. Text references and notes to the figures are given for guidance in fo
23、llowing these procedures. The standards and procedures near the bottom of each of the two figures are for use when economy of average sample size is an important consideration. The procedure presented in Figure 1 is applicable when production is continuous and there are more than 10 lots of product
24、undergoing inspection. Standards from the BS 6001 series are included here as “candidate” systems.BS 6000-2:2005 2 BSI 25 July 2005 Figure 2 is applicable when other conditions prevail, e.g. when there is no continuity of production, when the number of lots is 10 or fewer, when the product warranty
25、does not survive acceptance and/or when the presence of a small number of nonconforming items can result in a large loss. A final selection should be based on both the requirements of the situation and the resources of the inspection organization. In the process of selecting a sampling system, schem
26、e or plan, it is possible to find that the selection has indicated a system, scheme or plan that has one or more insurmountable deficiencies. This indicates that the process needs to be repeated. The cases in which only one method is appropriate are few. Iterative investigations usually identify two
27、 or more methods. The most economical and appropriate method should be chosen. 3 British Standards for acceptance sampling of lots by attributes 3.1 General This clause summarizes the salient features of each of the current British Standards concerned with acceptance sampling methods by attributes.
28、The summaries of the scope and application of the available British Standards should enable a user to select those standards which are most likely to suit a given purpose. The comparisons between the various British Standard acceptance sampling systems in these summaries are not enough to allow a fi
29、nal selection of a sampling system, scheme or plan to be made in a particular situation. Before this can be done, a number of factors need to be understood and considered. These factors are reviewed in Clause 4. 3.2 BS 6001-0: Introduction to the BS 6001 attribute sampling system This is a companion
30、 document to BS 6000-2 and can be read in conjunction with it, if desired, but this is not essential. It is not a source of sampling schemes or plans. Section 2 of BS 6001-0 is essentially an introduction to the sampling schemes employed in BS 6001-1, BS 6001-2 and BS 6001-3, but it treats the subje
31、ct in a general way. It contains explanations of terms, gives practical advice on sampling inspection and discusses some underlying concepts. Section 3 extends Section 2 and amplifies the introductory text and instructions contained in BS 6001-1, by giving detailed comments and examples to assist in
32、 using the procedures and tables that make up the BS 6001-1 system. 3.3 BS 6001-1: Sampling plans indexed by acceptance quality limit (AQL) for lot-by-lot inspection This presents a sampling system indexed by lot-size ranges, inspection levels and AQLs, and specifies sampling plans and procedures fo
33、r inspection by attributes of discrete items. It contains sampling plans for single, double and multiple sampling indexed by percent nonconforming and nonconformities per 100 items. The sampling system specified in BS 6001-1 is intended to employ tightened, normal and reduced inspection on a continu
34、ing series of lots to achieve customer protection while assuring the producer that, if quality is better than the AQL, acceptance will occur most of the time. The objective is to induce a supplier, through the economic and psychological pressure of potential non- acceptance, to maintain a process av
35、erage quality at least as good as the specified AQL, while at the same time providing an upper limit for the risk to the consumer of accepting the occasional lot of poor quality. The sampling plans in BS 6001-1 may also be used for the inspection of lots in isolation, but in this case the user is st
36、rongly advised to consult the operating characteristic curves (in Table 12 of the 1999 edition) to find a plan which will yield the desired protection. A much simpler procedure to follow in this type of situation is presented in BS 6001-2. 3.4 BS 6001-2: Sampling plans indexed by limiting quality (L
37、Q) for isolated lot inspection This establishes sampling plans indexed by limiting quality (LQ) and procedures for inspection that can be used when the switching rules of BS 6001-1 cannot be applied. The LQ is used to indicate the customer protection. Procedure A is used for single lots and procedur
38、e B for lots isolated from a continuing series. Both procedures treat the LQ as an indicator of the actual percentage nonconforming in the lots submitted, though they can also be used in cases where quality is expressed in non-conformities per 100 items.BS 6000-2:2005 BSI 25 July 2005 3 Procedure A
39、is used when both the supplier and the customer wish to regard the lot in isolation, but it is also used as the default procedure where there is no specific instruction to use procedure B. Procedure B is used when the supplier regards the lot as one of a continuing series, but the customer considers
40、 the lot received in isolation. The sampling plans employed permit a producer to maintain consistent procedures for customers, irrespective of whether the customers receive individual lots or a continuing series of lots. The manufacturer is concerned with all of the production but the individual cus
41、tomer only with the particular lot received. For procedure A, sampling plans are identified by their lot size and LQ; for procedure B, they are identified by lot size, LQ and inspection level. Procedure A includes sampling plans with an acceptance number of zero, but these are excluded from procedur
42、e B. Double and multiple sampling plans can be used as alternatives to single sampling plans in procedure B and for the non-zero acceptance number plans in procedure A. 3.5 BS 6001-3: Skip-lot sampling procedures This specifies a sampling system that extends the procedures specified in BS 6001-1. It
43、 provides generic attribute skip-lot sampling procedures for reducing the inspection effort on products submitted by suppliers who have demonstrated their ability to control, in an effective manner, all facets of product quality and to produce superior quality material consistently. However, these p
44、rocedures are proscribed for the inspection of product characteristics that involve the safety of personnel. The skip-lot programme uses the acceptance sampling plans specified in BS 6001-1 and is intended only for a continuing series of lots; it is not appropriate for isolated lots. All lots in a s
45、eries are expected to be of a similar quality and there should be no reason to believe that lots that have not been inspected are of a poorer quality than those that have. In a skip-lot sampling procedure, some lots in a series are accepted without inspection when the sampling results for a stated n
46、umber of immediately preceding lots meet stated criteria. The lots to be inspected are chosen randomly with a stated frequency, called the “skip-lot frequency”. 3.6 BS 6001-4: Sequential sampling plans for percent nonconforming This presents a sampling system that provides a wide range of sequential
47、 sampling plans indexed in terms of the consumers risk point (CRP) and the producers risk point (PRP). It also contains a sequential sampling system indexed by lot size ranges, inspection levels and AQL to supplement the system specified in BS 6001-1, including switching rules. (For the relationship
48、 between AQL, LQ, CRP and PRP, see Figure 1 and Figure 2.) In sequential sampling, items are selected randomly and inspected one after another, and a cumulative count is kept of the number of nonconforming items or number of nonconformities. The decision that “sentences” the lot as acceptable or as
49、not acceptable can occur at almost any stage and, for sequential sampling by attributes, depends on the number of items inspected and the cumulative number of nonconforming items or nonconformities found up to that point. BS 6001-4 provides procedures based on a sequential assessment of inspection results that may be used to induce the supplier to supply lots of a quality with a high probability of acceptance while maintaining an upper limit for the risk to the consumer of acce
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