Australian Standard. Sampling procedures and charts for inspection by variables for percent nonconforming AS ISO 3951:1989

Transcription

1 AS ISO 3951:1989 Australian Standard Sampling procedures and charts for inspection by variables for percent nonconforming

2 This Australian Standard was prepared by Committee QR/4, Statistical Quality Procedures. It was approved on behalf of the Council of Standards Australia on 14 February 1997 and published on 5 May The following interests are represented on Committee QR/4: Australian Organisation for Quality Australian Quality Council Bureau of Steel Manufacturers CSIRO - Division of Mathematics and Statistics National Association of Testing Authorities, Australia Quality Society of Australasia Statistical Society of Australia Queensland University of Technology University of Wollongong Review of Australian Standards. To keep abreast of progress in industry, Australian Standards are subject to periodic review and are kept up to date by the issue of amendments or new editions as necessary. It is important therefore that Standards users ensure that they are in possession of the latest editi on, and any amendments thereto. Full details of all Australian Standards and related publications will be found in the Standards Australia Catalogue of Publications; this information is supplemented each month by the magazine The Australian Standard, which subscribing members receive, and which gives details of new publications, new editions and amendments, and of withdrawn Standards. Suggestions for improvements to Australian Standards, addressed to the head office of Standards Australia, are welcomed. Notification of any inaccuracy or ambiguity found in an Australian Standard should be made without delay in order that the matter may be investigated and appropriate action taken. This Standard was issued in draft form for comment as DR Copyright STANDARDS AUSTRALIA Users of Standards are reminded that copyright subsists in all Standards Australia publications and software. Except where the Copyright Act allows and except where provided for below no publications or software produced by Standards Australia may be reproduced, stored in a retrieval system in any form or transmitted by any means without prior permission in writing from Standards Australia. Permission may be conditional on an appropriate royalty payment. Requests for permission and information on commercial software royalties should be directed to the head office of Standards Australia. Standards Australia will permit up to 10 percent of the technical content pages of a Standard to be copied for use exclusively in-house by purchasers of the Standard without payment of a royalty or advice to Standards Australia. Standards Australia will also permit the inclusion of its copyright material in computer software programs for no royalty payment provided such programs are used exclusively in-house by the creators of the programs. Care should be taken to ensure that material used is from the current edition of the Standard and that it is updated whenever the Standard is amended or revised. The number and date of the Standard should therefore be clearly identified. The use of material in print form or in computer software programs to be used commercially, with or without payment, or in commercial contracts is subject to the payment of a royalty. This policy may be varied by Standards Australia at any time.

3 AS Australian Standard Sampling procedures and charts for inspection by variables for percent nonconforming Originated as AS Second edition PUBLISHED BY STANDARDS AUSTRALIA (STANDARDS ASSOCIATION OF AUSTRALIA) 1 THE CRESCENT, HOMEBUSH, NSW 2140 ISBN

4 ii PREFACE This Standard was prepared by the Joint Standards Australia/Standards New Zealand Committee QR/4 on Statistical Quality Procedures as an Australian Standard. The Standard is identical with and has been reproduced from ISO 3951:1989, Sampling procedures and charts for inspection by variables for percent nonconforming. This Standard supersedes AS , which was based on ISO 3951:1981 and therefore is consistent with the changes that have occurred in the evolution of ISO This Standard continues to be complimentary to AS 1199 and AS 1399, since it provides alternative methods of sampling inspection indexed in terms of acceptable quality levels (AQL). It should be noted, however, that the variables sampling plans given in this Standard can be more economical in that they require relatively smaller sample sizes (for a particular value of AQL) since more effective use can be made of data from the sample results if the measurements can be assumed to follow a normal distribution. The theory of this Standard assumes independence of the data. Hence this Standard should not be applied where the data may be highly auto-correlated, either as a result of auto-correlation of the production process or the sampling process or both. As this Standard is reproduced from an International Standard, the following applies: (a) Its number does not appear on each page of text and its identity is shown only on its cover and title page. (b) In the source text, this international Standard should read this Australian Standard. (c) A full point substitutes for a comma when referring to a decimal point. For the purposes of this Standard, the ISO references should be modified by replacing references to ISO Standards with references to Australian Standards, as follows: Reference to International Standard Australian Standard ISO AS 2854 Statistical interpretation of data Techniques of estimation and tests relating to means and variances 2859 Sampling procedures for inspection by attributes Part 0: Introduction to the ISO 2859 attribute sampling system Part 1:Sampling plans indexed by acceptable quality level (AQL) for lot-by-lot inspection Part 2:Sampling plans indexed by limiting quality (LQ) for isolated lot inspection Part 3:Skip-lot sampling procedures 3534 Statistics Vocabulary and symbols 5725 Accuracy (trueness and precision) of measurement methods and results 1399 Guide to AS 1199Sampling procedures and tables for inspection by attributes 1199 Sampling procedures and tables for inspection by attributes NOTE: The ISO Standard also makes reference to ISO This is still at the Draft International Standard stage and refers to ISO/DIS 5479, issued 16 February 1996.

5 iii CONTENTS Page Section one : General Scopeandfieldofapplication References Definitionsandsymbols AcceptableQualityLevel(AQL) Switching rules for normal, tightened and reduced inspection RelationtoISO Non-continuous production and operating characteristic curves Section two : Choice of sampling plan Planning Choice between variables and attributes Choiceofmethod ChoiceofinspectionlevelandAQL Choiceofsamplingplan... 7 Section three : Operation of a variables sampling plan Preliminaryoperations Standard procedure for s method Standard procedure for σ method Procedure during continuing inspection Normalityandoutliers Records Operationofswitchingrules Discontinuation and resumption of inspection Acceptance curves for tightened and reduced inspection Switching to σ method... 16

6 iv Page Section four : Tables and diagrams Tables I-A Sample size code letters and inspection levels I-B II-A II-B II-C Sample size code letters and sample sizes for normal inspection Single sampling plans for normal inspection (master table): s method Single sampling plans for tightened inspection (master table) : s method Single sampling plans for reduced inspection (master table) : s method III-A Single sampling plans for normal inspection (master table) : σ method III-B III-C IV-s IV-σ Single sampling plans for tightened inspection (master table) : σ method Single sampling plans for reduced inspection (master table) : σ method Values of f s for maximum sample standard deviation (MSSD) : s method Values of f σ, for maximum process standard deviation (MPSD): σ method V (V-B to V-P) Tables and operating characteristic curves for single samplingplans(samplesizecodelettersbtop) VI-A VI-B Correspondence between tightened and normal inspectionplans Correspondence between reduced and normal inspectionplans Diagrams A Sample size code letters of standard sampling plans for specified qualities at 95 % and 10 % probabilities of acceptance s-d to s-p Acceptance curves for combined double specification limits : s method σ-c to σ-p Acceptance curves for combined double specification limits : σ method Annexes A Procedures for obtaining s and σ B Statisticaltheory C Sampling plans for R method D Graph paper for the s method Bibliography...107

7 1 AUSTRALIAN STANDARD Sampling procedures and charts for inspection by variables for percent nonconforming Section one : General 1 Scope and field of appl icat ion 1.1 Scope This International Standard establishes sampling plans and procedures for inspection by variables. It is complementary to ISO When specified by the responsible authority, both this International Standard and ISO 2859 may be referenced in a product or process specification, contract, inspection instructions, or other documents, and the provisions set forth therein shall govern. The responsible authority shall be designated in one of the above documents The object of the methods laid down in this International Standard is to ensure that lots of an acceptable quality have a high probability of acceptance and that the probability of not accepting inferior lots is as high as possible In common with ISO the percentage of nonconforming products in the lots is used to define the quality of these lots and of the production process in question. 1.2 Field of application This International Standard is primarily designed for use under the following conditions : a) where the inspection procedure is to be applied to a continuous series of lots of discrete products all supplied by one producer using one production process. If there are different producers, this International Standard shall be applied 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. If several such characteristics are of importance. this International Standard shall be applied to each separately; c) where production is stable (under statistical control) and the quality characteristic x is distributed according to a normal distribution or a close approximation to the normal distribution; 1) At present at the stage of draft. d) where a contract or standard defines an upper specification limit U, a lower specification limit L, or both; a product is qualifi ed as nonconforming when its measured quality characteristic x satisfies one of the following inequaliti es: x > U... (1) x < L... (2) either x > U or x < L... (3) Inequalities (1) and (2) are called cases with a single specification limi t, and (3) a case with double specification limits. In this last situation a further distinction is made between separate or combined double limit s according to whether the AQL is applied to each limit separately or to both limits combined (see clause 4). 2 References ISO 2854, Statistical interpretation of data Techniques of estimation and tests relating to means and variances. ISO 2859, Sampling procedures and tables for inspection by attributes. ISO 3534, Statistics Vocabulary and symbols. ISO 5479, Normality tests. 1) ISO 5725, Precision of test methods Determination of repeatabillty and reproducibility for a standard test method by inter-laboratory tests. 3 Definitions and symbols 3.1 Definitions For the purposes of this International Standard, the definitions given in ISO 3534 and ISO 2859, together with the following, apply. COPYRIGHT

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