SANS 3951-5-2008 Sampling procedures for inspection by variables Part 5 Sequential sampling plans indexed by acceptance quality limit (AQL) for inspection by variables (known stand.pdf

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1、 Collection of SANS standards in electronic format (PDF) 1. Copyright This standard is available to staff members of companies that have subscribed to the complete collection of SANS standards in accordance with a formal copyright agreement. This document may reside on a CENTRAL FILE SERVER or INTRA

2、NET SYSTEM only. Unless specific permission has been granted, this document MAY NOT be sent or given to staff members from other companies or organizations. Doing so would constitute a VIOLATION of SABS copyright rules. 2. Indemnity The South African Bureau of Standards accepts no liability for any

3、damage whatsoever than may result from the use of this material or the information contain therein, irrespective of the cause and quantum thereof. ISBN 978-0-626-21187-5 SANS 3951-5:2008 Edition 1 ISO 3951-5:2006 Edition 1SOUTH AFRICAN NATIONAL STANDARD Sampling procedures for inspection by variable

4、s Part 5: Sequential sampling plans indexed by acceptance quality limit (AQL) for inspection by variables (known standard deviation) This national standard is the identical implementation of ISO 3951-5:2006 and is adopted with the permission of the International Organization for Standardization. Pub

5、lished by Standards South Africa 1 dr lategan road groenkloof private bag x191 pretoria 0001 tel: 012 428 7911 fax: 012 344 1568 international code + 27 12 www.stansa.co.za Standards South Africa This standard may only be used and printed by approved subscription and freemailing clients of the SABS.

6、SANS 3951-5:2008 Edition 1 ISO 3951-5:2006 Edition 1 Table of changes Change No. Date Scope National foreword This South African standard was approved by National Committee StanSA TC 169, Applications of statistical methods, in accordance with procedures of Standards South Africa, in compliance with

7、 annex 3 of the WTO/TBT agreement. This SANS document was published in April 2008. This standard may only be used and printed by approved subscription and freemailing clients of the SABS. Reference number ISO 3951-5:2006(E) ISO 2006INTERNATIONAL STANDARD ISO 3951-5 First edition 2006-03-01 Sampling

8、procedures for inspection by variables Part 5: Sequential sampling plans indexed by acceptance quality limit (AQL) for inspection by variables (known standard deviation) Rgles dchantillonnage pour les contrles par mesures Partie 5: Plans dchantillonnage squentiels indexs daprs la limite dacceptation

9、 de qualit (LAQ) pour linspection par variables (cart- type connu) SANS 3951-5:2008This standard may only be used and printed by approved subscription and freemailing clients of the SABS.ISO 3951-5:2006(E) PDF disclaimer This PDF file may contain embedded typefaces. In accordance with Adobes licensi

10、ng policy, this file may be printed or viewed but shall not be edited unless the typefaces which are embedded are licensed to and installed on the computer performing the editing. In downloading this file, parties accept therein the responsibility of not infringing Adobes licensing policy. The ISO C

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12、taken to ensure that the file is suitable for use by ISO member bodies. In the unlikely event that a problem relating to it is found, please inform the Central Secretariat at the address given below. ISO 2006 All rights reserved. Unless otherwise specified, no part of this publication may be reprodu

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14、 01 11 Fax + 41 22 749 09 47 E-mail copyrightiso.org Web www.iso.org Published in Switzerland ii ISO 2006 All rights reservedSANS 3951-5:2008This standard may only be used and printed by approved subscription and freemailing clients of the SABS.ISO 3951-5:2006(E) ISO 2006 All rights reserved iiiCont

15、ents Page Foreword iv Introduction v 1 Scope . 1 2 Normative references . 2 3 Terms and definitions. 2 4 Symbols . 6 5 Acceptance quality limit (AQL) 7 6 Switching rules for normal, tightened and reduced inspection 7 7 Planning. 9 8 Choice between variables and attributes. 9 9 Choice of inspection l

16、evel and AQL . 9 10 Preliminary operations. 10 11 Sampling plans . 10 12 Further information. 21 13 Examples . 22 14 Tables. 24 Annex A (normative) Sampling plans for normal, tightened and reduced inspection 25 Annex B (normative) Critical values for maximum standard deviation 29 Annex C (informativ

17、e) Statistical characteristics 32 Bibliography . 36 SANS 3951-5:2008This standard may only be used and printed by approved subscription and freemailing clients of the SABS.ISO 3951-5:2006(E) iv ISO 2006 All rights reservedForeword ISO (the International Organization for Standardization) is a worldwi

18、de 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 tha

19、t 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. International Standards are drafted in a

20、ccordance 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 technical committees are circulated to the member bodies for voting. Publication as an International Standard re

21、quires approval by at least 75 % of the member bodies casting a vote. 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 3951-5 was prepared by Tech

22、nical Committee ISO/TC 69, Applications of statistical methods, Subcommittee SC 5, Acceptance sampling. This edition cancels and replaces Annex A of ISO 8423:1991, which has been technically revised to greatly improve its compatibility with the sampling systems in ISO 3951-1. ISO 3951 consists of th

23、e following parts, under the general title Sampling procedures for inspection by variables: Part 1: Specification for single sampling plans indexed by acceptance quality limit (AQL) for lot-by-lot inspection for a single quality characteristic and a single AQL Part 5: Sequential sampling plans index

24、ed by acceptance quality limit (AQL) for inspection by variables (known standard deviation) The following parts are under preparation: Part 2: General specification for single sampling plans indexed by acceptance quality limit (AQL) for lot-by-lot inspection of independent quality characteristics Pa

25、rt 3: Double sampling schemes indexed by acceptance quality limit (AQL) for lot-by-lot inspection SANS 3951-5:2008This standard may only be used and printed by approved subscription and freemailing clients of the SABS.ISO 3951-5:2006(E) ISO 2006 All rights reserved vIntroduction In contemporary prod

26、uction processes, quality is often expected to reach such high levels that the number of nonconforming items is reported in parts per million. Under such circumstances, popular acceptance sampling plans by attributes, such as those presented in ISO 2859-1, require prohibitively large sample sizes. W

27、hen it is possible to apply acceptance sampling plans by variables, such as those presented in ISO 3951-1, the sample sizes are much smaller. However, especially in the case of acceptance of a product of extremely high quality, those sample sizes are still too large. Therefore, there is a need to ap

28、ply standardized statistical procedures that require the smallest possible sample sizes. Sequential sampling plans are the only statistical procedures that satisfy that need. It has been mathematically proved that among all possible sampling plans having similar statistical properties, the sequentia

29、l sampling plan has the smallest average sample size. Therefore, there is a strong need to present sequential sampling plans which are statistically equivalent to the commonly used acceptance sampling plans from ISO 3951-1, but which require significantly smaller average sample sizes. The principal

30、advantage of sequential sampling plans is the reduction in the average sample size. The average sample number is the average of all the sample sizes that may occur under a sampling plan for a given lot or process quality level. The use of sequential sampling plans leads to a smaller average sample n

31、umber than single sampling plans having the equivalent operating characteristic. For the sequential sampling plans in this part of ISO 3951, a curtailment rule has been introduced involving an upper limit of 1,5 n 0on the actual number of items to be inspected, where n 0is the sample size of the cor

32、responding single sampling plan. Other factors that should be taken into account are as follows: a) Complexity The rules of a sequential sampling plan are more easily misunderstood by inspectors than the simple rules for a single sampling plan. b) Variability in the amount of inspection As the actua

33、l number of items inspected for a particular lot is not known in advance, the use of sequential sampling plans brings about various organizational difficulties. For example, scheduling of inspection operations may be difficult. c) Ease of drawing sample items If drawing sample items is at different

34、times rather difficult, the reduction in the average sample size by sequential sampling plans may be cancelled out by the increased sampling cost. d) Duration of test If the test of a single item is of long duration and a number of items can be tested simultaneously, sequential sampling plans are mu

35、ch more time-consuming than the corresponding single sampling plans. e) Variability of quality within the lot If the lot consists of two or more sublots from different sources and if there is likely to be any substantial difference between the qualities of the sublots, drawing of a random sample und

36、er a sequential sampling plan is more awkward than under the corresponding single sampling plan. The balance between the advantage of a smaller average sample number of the sequential sampling plan and the above disadvantages leads to the conclusion that sequential sampling plans are suitable only w

37、hen inspection of individual items is costly in comparison with inspection overheads. SANS 3951-5:2008This standard may only be used and printed by approved subscription and freemailing clients of the SABS.ISO 3951-5:2006(E) vi ISO 2006 All rights reservedThe choice between single and sequential sam

38、pling plans should be made before the inspection of a lot is started. During inspection of a lot, it is not permitted to switch from one type of plan to another, because the operating characteristics of the plan may be drastically changed if the actual inspection results influence the choice of acce

39、ptability criteria. Although a sequential sampling plan is on average much more economical than the corresponding single sampling plan, it may occur, during inspection of a particular lot, that acceptance and non-acceptance comes at a very late stage because the cumulative leeway (the statistic used

40、 for the determination of lot acceptability) remains between the acceptance value and the rejection value for a long time. With the graphical method, this corresponds to the random progress of the step-wise linear curve remaining in the indecision zone. In order to alleviate this disadvantage, the s

41、ample size curtailment value is set before the inspection of a lot is started, and inspection terminates if the cumulative sample size reaches the curtailment value, n t , without determination of lot acceptability. The acceptance and non-acceptance of the lot is then determined using the curtailmen

42、t acceptance and rejection values. For sequential sampling plans in common use, curtailment usually represents a deviation from their intended usage, leading to a distortion of their operating characteristics. In this part of ISO 3951, however, the operating characteristics of the sequential samplin

43、g plans have been determined with curtailment taken into account, so curtailment is an integral component of the provided plans. SANS 3951-5:2008This standard may only be used and printed by approved subscription and freemailing clients of the SABS.INTERNATIONAL STANDARD ISO 3951-5:2006(E) ISO 2006

44、All rights reserved 1Sampling procedures for inspection by variables Part 5: Sequential sampling plans indexed by acceptance quality limit (AQL) for inspection by variables (known standard deviation) 1 Scope This part of ISO 3951 specifies a system of sequential sampling plans (schemes) for lot-by-l

45、ot inspection by variables. The schemes are indexed in terms of a preferred series of acceptance quality limit (AQL) values, ranging from 0,01 to 10, which are defined in terms of percent nonconforming items. The schemes of ISO 3951 are intended to induce a supplier through the economic and psycholo

46、gical pressure of lot non-acceptance to maintain a process average at least as good as the specified AQL value, while at the same time providing an upper limit for the risk to the consumer of accepting the occasional poor lot. The schemes are designed to be applied to a continuing series of lots, th

47、at is, a series long enough to allow the switching rules (Clause 6) to be applied. These switching rules provide: automatic protection to the consumer (by means of a switch to tightened inspection or discontinuation of sampling inspection) should a deterioration in quality be detected; an incentive

48、(at the discretion of the responsible authority) to reduce inspection costs (by means of a switch to reduced inspection) should consistently good quality be achieved. This part of ISO 3951 is designed for use under the following conditions: a) where the inspection procedure is to be applied to a con

49、tinuing series of lots of discrete products all supplied by one producer using one production process. If there are different producers or production processes, apply this part of ISO 3951 to each one separately; b) where only a single quality characteristic, x, of these products is taken into consideration, which must be measurable on a continuous scale; c) where the uncertainty of the measurement system is negligible with respect to the production process standard deviation; d) where production is stable (under statistical