ISO 3951-5-2006 Sampling procedures for inspection by variables - Part 5 Sequential sampling plans indexed by acceptance quality limit (AQL) for inspection by v.pdf

1、 Reference number ISO 3951-5:2006(E) ISO 2006INTERNATIONAL STANDARD ISO 3951-5 First edition 2006-03-01 Sampling procedures for inspection by variables Part 5: Sequential sampling plans indexed by acceptance quality limit (AQL) for inspection by variables (known standard deviation) Rgles dchantillon

2、nage pour les contrles par mesures Partie 5: Plans dchantillonnage squentiels indexs daprs la limite dacceptation de qualit (LAQ) pour linspection par variables (cart- type connu) ISO 3951-5:2006(E) PDF disclaimer This PDF file may contain embedded typefaces. In accordance with Adobes licensing poli

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7、Fax + 41 22 749 09 47 E-mail copyrightiso.org Web www.iso.org Published in Switzerland ii ISO 2006 All rights reservedISO 3951-5:2006(E) ISO 2006 All rights reserved iiiContents Page Foreword iv Introduction v 1 Scope . 1 2 Normative references . 2 3 Terms and definitions. 2 4 Symbols . 6 5 Acceptan

8、ce 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 level and AQL . 9 10 Preliminary operations. 10 11 Sampling plans . 10 12 Further information. 21 13 Examples . 22 14 Tables. 24

9、 Annex A (normative) Sampling plans for normal, tightened and reduced inspection 25 Annex B (normative) Critical values for maximum standard deviation 29 Annex C (informative) Statistical characteristics 32 Bibliography . 36 ISO 3951-5:2006(E) iv ISO 2006 All rights reservedForeword ISO (the Interna

10、tional 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 be

11、en 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 stan

12、dardization. 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 technical committees are circulated to the member bodies fo

13、r voting. Publication as an International Standard requires 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 al

14、l such patent rights. ISO 3951-5 was prepared by Technical 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 sa

15、mpling systems in ISO 3951-1. 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 characteristic and

16、 a single AQL Part 5: Sequential sampling plans indexed 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

17、 inspection of independent quality characteristics Part 3: Double sampling schemes indexed by acceptance quality limit (AQL) for lot-by-lot inspection ISO 3951-5:2006(E) ISO 2006 All rights reserved vIntroduction In contemporary production processes, quality is often expected to reach such high leve

18、ls 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. When it is possible to apply acceptance sampling plans by variables,

19、 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 apply standardized statistical procedures that require the smallest p

20、ossible 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 sequential sampling plan has the smallest average sample size. Therefore, th

21、ere 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 advantage of sequential sampling plans is the reduction in the aver

22、age 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 number than single sampling plans having the equivalent operating ch

23、aracteristic. 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 corresponding single sampling plan. Other factors that should be taken

24、 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 actual number of items inspected for a particular lot is not known in ad

25、vance, 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 times rather difficult, the reduction in the average sample size by

26、 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 much more time-consuming than the corresponding single sampling plans

27、. 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 under a sequential sampling plan is more awkward than under the corres

28、ponding 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 when inspection of individual items is costly in comparison with ins

29、pection overheads. ISO 3951-5:2006(E) vi ISO 2006 All rights reservedThe choice between single and sequential sampling 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 cha

30、racteristics of the plan may be drastically changed if the actual inspection results influence the choice of acceptability 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

31、, that acceptance and non-acceptance comes at a very late stage because the cumulative leeway (the statistic used 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 progre

32、ss of the step-wise linear curve remaining in the indecision zone. In order to alleviate this disadvantage, the sample 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 determi

33、nation of lot acceptability. The acceptance and non-acceptance of the lot is then determined using the curtailment 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 oper

34、ating characteristics. In this part of ISO 3951, however, the operating characteristics of the sequential sampling plans have been determined with curtailment taken into account, so curtailment is an integral component of the provided plans. INTERNATIONAL STANDARD ISO 3951-5:2006(E) ISO 2006 All rig

35、hts 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-lot insp

36、ection 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 psychological p

37、ressure 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, that is,

38、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 (at the

39、 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 continuing

40、 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 consideratio

41、n, 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 control) and the quality characteristic, x, is distributed according to a no

42、rmal distribution (or a close approximation to the normal distribution) or a distribution which may be mathematically transformed to a normal distribution; e) where the standard deviation of the quality characteristic, x, is known; CAUTION The procedures in this part of ISO 3951 are not suitable for

43、 application to lots that have been screened previously for nonconforming items. f) where a contract or standard defines an upper specification limit, U, a lower specification limit, L, or both; an item is qualified as conforming if and only if its measured quality characteristic, x, satisfies the a

44、ppropriate one of the following inequalities: ISO 3951-5:2006(E) 2 ISO 2006 All rights reserved1) x u U (i.e. the single upper specification limit is not violated); 2) x W L (i.e. the single lower specification limit is not violated); 3) L u x u U (i.e. the upper and lower double specification limit

45、s are not violated). In this part of ISO 3951, it is assumed that, where double specification limits apply, conformance to both specification limits is either equally important to the integrity of the product or is considered separately for both specification limits. In the first case, it is appropr

46、iate to apply a single AQL to the combined percentage of product outside the two specification limits. This is referred to as a combined AQL requirement. In the second case, separate AQLs apply to nonconformity beyond each of the limits, and this is referred to as a separate AQL requirement. In this

47、 part of ISO 3951, the acceptability of a lot is implicitly determined from an estimate of the percentage of nonconforming items in the process, based on a random sample of items from the lot. As such, it is not applicable for judging the acceptability of isolated lots or short series of lots. Refer

48、 to ISO 2859-2 for applicable sampling plans in this case. 2 Normative references The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (in

49、cluding any amendments) applies. ISO 2859-1:1999, Sampling procedures for inspection by attributes Part 1: Sampling schemes indexed by acceptance quality limit (AQL) for lot-by-lot inspection ISO 3534-1, Statistics Vocabulary and symbols Part 1: General statistical terms and terms used in probability ISO 3534-2, Statistics Vocabulary and symbols Part 2: Applied statistics ISO 3951-1:2005, Sampling procedures for inspection by variables Part 1: Specification for single sampling plans indexed by acceptance quality