ISO 3951-3-2007 Sampling procedures for inspection by variables - Part 3 Double sampling schemes indexed by acceptance quality limit (AQL) for lot-by-lot inspec.pdf

1、 Reference number ISO 3951-3:2007(E) ISO 2007INTERNATIONAL STANDARD ISO 3951-3 First edition 2007-05-01 Sampling procedures for inspection by variables Part 3: Double sampling schemes indexed by acceptance quality limit (AQL) for lot-by-lot inspection Rgles dchantillonnage pour les contrles par mesu

2、res Partie 3: Plans dchantillonnage doubles pour le contrle lot par lot, indexs daprs le niveau de qualit acceptable (NQA) ISO 3951-3:2007(E) PDF disclaimer This PDF file may contain embedded typefaces. In accordance with Adobes licensing policy, this file may be printed or viewed but shall not be e

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7、 copyrightiso.org Web www.iso.org Published in Switzerland ii ISO 2007 All rights reservedISO 3951-3:2007(E) ISO 2007 All rights reserved iii Contents Page Foreword. v Introduction . vi 1 Scope. 1 2 Normative references. 2 3 Terms and definitions. 2 4 Symbols and abbreviations . 7 5 Acceptance quali

8、ty limit (AQL) 9 6 Switching rules for normal, tightened and reduced inspection 10 7 Relation to ISO 2859-1 10 8 Limiting quality protection. 11 9 Planning. 12 10 Choice between variables and attributes. 12 11 Choice of method 13 12 Choice between single and double sampling plans . 13 13 Choice of i

9、nspection level and AQL . 14 14 Choice of sampling scheme 14 15 Preliminary operations. 15 16 Standard univariate “s” method procedures. 16 17 Standard univariate “ ” method procedures. 25 18 Procedure during continuing inspection . 32 19 Normality and outliers 32 20 Records 32 21 Operation of switc

10、hing rules . 33 22 Discontinuation and resumption of inspection . 33 23 Switching between the “s” and “ ” methods 34 Annex A (informative) Standard multivariate “s” method procedures for double sampling with independent quality characteristics . 72 Annex B (informative) Standard multivariate “ ” met

11、hod procedures for double sampling with independent quality characteristics . 74 Annex C (informative) Standard multivariate combined “s” and “ ” method procedures for double sampling with independent quality characteristics 76 Annex D (informative) Location of text on key features. 78 Annex E (norm

12、ative) Estimating the process fraction nonconforming 81 Annex F (informative) Form k “s” method single sampling plans matched to the corresponding single sampling plans by attributes 87 ISO 3951-3:2007(E) iv ISO 2007 All rights reservedAnnex G (informative) Form k “ ” method single sampling plans ma

13、tched to the corresponding single sampling plans by attributes 91 Annex H (informative) Average sample sizes for double sampling by variables: “s” method . 95 Annex I (informative) Producers risks for the “s” method 98 Annex J (informative) Tabulated operating characteristics for double sampling pla

14、ns with known process standard deviation . 100 Annex K (informative) Ratios of ASSIs of double sampling plans under normal inspection to the sample size of the corresponding single sampling plan by variables 107 Annex L (informative) Ratios of the ASSIs of double sampling plans by variables to the A

15、SSIs of corresponding plans by attributes 109 Annex M (informative) Design methodology . 112 Bibliography . 113 ISO 3951-3:2007(E) ISO 2007 All rights reserved v Foreword ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies (ISO member bodie

16、s). 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 n

17、on-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 accordance with the rules given in the ISO/IEC Directives, Pa

18、rt 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 requires approval by at least 75 % of the member bodies castin

19、g 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 3951-3 was prepared by Technical Committee ISO/TC 69, Applications of statistical metho

20、ds, Subcommittee SC 5, Acceptance sampling. ISO 3951 consists of the 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 char

21、acteristic and a single AQL Part 2: General specification for single sampling plans indexed by acceptance quality limit (AQL) for lot- by-lot inspection of independent quality characteristics Part 3: Double sampling schemes indexed by acceptance quality limit (AQL) for lot-by-lot inspection Part 5:

22、Sequential sampling plans indexed by acceptance quality limit (AQL) for inspection by variables (known standard deviation) The following part is under preparation: Part 4: Procedures for assessment of declared quality levels ISO 3951-3:2007(E) vi ISO 2007 All rights reservedIntroduction Inspection b

23、y variables for percentage nonconforming items, as described in this part of ISO 3951, includes several possible modes, the combination of which leads to a presentation which may appear quite complicated to the user: a) procedures for unknown process standard deviation (the “s” method), or procedure

24、s for where the process standard deviation is originally unknown then estimated with fair precision, or known since the start of inspection (the “ ” method); b) a single specification limit, or double specification limits with separate, combined or complex control; c) normal inspection, tightened in

25、spection or reduced inspection; d) Form k plans and Form p* plans; e) a single quality characteristic (the univariate case) or a number of unrelated quality characteristics (the multivariate independent case). The text has been arranged so that the simpler procedures may be implemented without neces

26、sarily understanding the more complicated procedures. The main text of this part of ISO 3951 is confined to the univariate case. The multivariate independent cases are treated separately in Annex A for the “s” method, in Annex B for the “ ” method and in Annex C for combined “s” method and “ ” metho

27、d procedures. Annex D facilitates the use of the main text of the standard by directing the user to the clauses and tables concerning any univariate situation with which he might be confronted; it only deals with Clauses 16, 17, 21, 22 and 23 and, in every case, it is necessary to have read Clauses

28、1 to 15 and Clauses 18 to 20 first. This part of ISO 3951 is complementary to the double sampling plans and procedures of ISO 2859-1. When specified by the responsible authority, it would be valid to reference both ISO 3951-3 and ISO 2859-1 in a product specification, a contract, inspection instruct

29、ions, or other documents, and the provisions set forth therein shall govern. The “responsible authority” can then be designated in one of these documents. In all parts of ISO 3951: the acronym AQL stands for “acceptance quality limit” rather than “acceptable quality level”, in order to more accurate

30、ly reflect its function; procedures are given for the case where the process standard is unknown (the “s” method) and for the case where it may be presumed to be known (the “ ” method); the sampling plans have been chosen so that their operating characteristic curves closely match those of the corre

31、sponding single sampling plans in ISO 2859-1; minimal statistical theory has been given (it being planned ultimately to provide this in a guidance document to sampling procedures for inspection by variables); text, charts and tables that are only informative have been consigned to annexes wherever p

32、racticable. In none of the parts have methods been given based on the sample range, now that the availability of computers and calculators with a standard deviation function key is so widespread. Data for acceptance sampling by variables is often substantially more expensive to acquire than data for

33、 sampling by attributes, and the “s” method makes more efficient use of these data. ISO 3951-3:2007(E) ISO 2007 All rights reserved vii The coverage of ISO 3951-1 is constrained to the case of a single, normally distributed, quality characteristic with a single class of nonconformity, and includes t

34、he case of combined control of double specification limits. ISO 3951-2 provides a more comprehensive treatment of single sampling plans by variables, including procedures for separate and complex control of double specification limits. Procedures are also given for multiple independent quality chara

35、cteristics and/or multiple AQLs. ISO 3951-3 provides plans for double sampling by variables, which on average provide substantial savings of inspection effort in comparison with plans for single sampling by variables. The savings are achieved by first selecting from the lot and inspecting a random s

36、ample that is typically nearly 40 % smaller than that of the corresponding single sampling plan. If these inspection results satisfy an acceptance criterion, an immediate decision is made to accept the lot without further inspection. Alternatively, if the inspection results satisfy a non-acceptance

37、criterion, an immediate decision not to accept the lot is made without further inspection. Thus, when quality is very good or very poor, the saving in inspection effort can amount to nearly 40 %. Only when the inspection results from the first sample are equivocal is a second random sample, of the s

38、ame size as the first, selected; the acceptability of the lot is then resolved by combining the results of the first and second samples and determining whether they satisfy a second acceptance criterion. This publication does not purport to include all the necessary provisions of a contract. Users a

39、re responsible for its correct application. INTERNATIONAL STANDARD ISO 3951-3:2007(E) ISO 2007 All rights reserved 1 Sampling procedures for inspection by variables Part 3: Double sampling schemes indexed by acceptance quality limit (AQL) for lot-by-lot inspection 1 Scope This part of ISO 3951 speci

40、fies an acceptance sampling system of double sampling schemes for inspection by variables for percent nonconforming. It is indexed in terms of the acceptance quality limit (AQL). The objectives of the methods laid down in this part of ISO 3951 are to ensure that lots of acceptable quality have a hig

41、h probability of acceptance and that the probability of non-accepting inferior lots is as high as practicable. This is achieved by means of the switching rules, which provide automatic protection to the consumer (by means of a switch to tightened inspection or discontinuation of sampling inspection)

42、 should a deterioration in quality be detected, and an incentive (at the discretion of the responsible authority) to reduce inspection costs (by means of a switch to a smaller sample size) should consistently good quality be achieved. In this part of ISO 3951, the acceptability of a lot is implicitl

43、y or explicitly determined from an estimate of the percentage of nonconforming items in the process, based on either one or two random samples of items from the lot. This part of ISO 3951 is primarily designed for use under the following conditions: a) where the inspection procedure is to be applied

44、 to a continuing 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 the items of product have a single quality characteristic (for multip

45、le quality characteristics, see informative Annexes A, B and C); c) where the quality characteristic is measurable on a continuous scale; d) where the measurement error is negligible (i.e. with a standard deviation of no more than 10 % of the corresponding process standard deviation); e) where produ

46、ction is stable (under statistical control) and the quality characteristic is distributed, at least to a close approximation, according to a normal distribution; CAUTION The procedures in this part of ISO 3951 are not suitable for application to lots that have been screened previously for nonconform

47、ing items. f) where the possibility of having to select and inspect a second sample is administratively acceptable; ISO 3951-3:2007(E) 2 ISO 2007 All rights reservedg) where a contract or standard defines an upper specification limit U, a lower specification limit L or both on the quality characteri

48、stic. An item is deemed to conform if its measured quality characteristic x satisfies the appropriate one of the following inequalities: 1) x W L (i.e. the lower specification limit is not violated); 2) x u U (i.e. the upper specification limit is not violated); 3) x W L and x u U (i.e. neither the

49、lower nor the upper specification limit is violated). NOTE Inequalities 1) and 2) are called cases with a “single specification limit”, and 3) is the case with “double specification limits”. For double specification limits, a further distinction is made between combined control, separate control and complex control, as follows: combined control is where a single AQL applies to nonconformity beyond both limits; separate control is where separate AQLs apply to nonconformity beyond each of the limit