1、ASQC S2 95 m 0759506 0001b ObT m ANSVASQC S2-1995 AMERICAN NATIONAL STANDARD Introduction to Attribute Sampling AMERICAN SOCIETY FOR QUALITY CONTROL 61 1 EAST WISCONSIN AVENUE MI LWAUKEE, WI SCONS1 N 53202 ASQC S2 95 075950b 000LBb TTb ANSVASQC S2-1995 AMERICAN NATIONAL STANDARD Introduction to Attr
2、ibute Sampling Prepared by American Society for Quality Control Standards Committee for American National standards Committee Z- 1 on Quality Assurance An American National Standard Approved on October 12, 1995 American National Standards: An American National Standard implies a consensus of those s
3、ubstantially concerned with its scope and provisions. An American National Standard is intended as a guide to aid the manufacturer, the con- sumer, and the general public. The existence of an American National Standard does not in any respect preclude any- one, whether he or she has approved the sta
4、ndard or not, from manufacturing, purchasing, or using products, process- es, or procedures not conforming to the standard. American National Standards are subject to periodic review and users are cautioned to obtain the latest edition. Caution Notice: This American National Standard may be revised
5、or withdrawn at any time. The procedures of the American National Standards Institute require that action be taken to reaffirm, revise, or withdraw this standard no later than five years from the date of publication. Purchasers of American National Standards may receive current information on all st
6、andards by calling or writing the American National Standards Institute. ASQC S2 95 m 075950b OOOL870 718 m O 1995 by ASQC All rights reserved. No part of this book may be reproduced in any form or by any means, electronic, mechanical, pho- tocopying, recording, or otherwise, without the prior writt
7、en permission of the publisher. ASQC Mission: To facilitate continuous improvement and increase customer satisfaction by identifying, communicat- ing, and promoting the use of quality principles, concepts, and technologies; and thereby be recognized throughout the world as the leading authority on,
8、and champion for, quality. 109 8 7 6 5 4 3 2 1 Printed in the United States of America Printed on acid-free paper Published by: ASQC 611 East Wisconsin Avenue Milwaukee, Wisconsin 53202 ASQC S2 95 075950b 0001871 b54 m ANSUASQC S2-1995 Contents Section 1: General Introduction to Acceptance Sampling
9、1 .o 1.1 1.2 1.3 1.4 1.5 I 1.6 1.7 1.8 1.9 1.10 1.11 1.12 1.13 1.14 1.15 1.16 1.17 1.18 1.19 1.20 1.21 1.22 1.23 1.24 1.25 1.26 Introduction Scope . Objective of sampling inspection . Acceptance sampling 1.3.1 Statistical sampling . 1.3.2 Ad hoc sampling (sampling with no statistical basis) 1.3.3 10
10、0% inspection of items 1.3.4 Other sampling practices Attributes or variables inspection . Lotinspection 1.5.1 Lot 1.5.2 Lotsize Sequence or isolated-lot inspection . 1.6.1 Lot-by-lot inspection 1.6.2 Process improvement . Large vs . small lots . 1.6.5 Isolated-lot inspection . Acceptablequalityleve
11、l 1.7.2 Setting an AQL Normal and tightened inspection . Switching rules . Average outgoing quality and its limit . Item: unit of product . Nonconformity and nonconforming item . 1.15.1 Failure to conform 1.15.2 Nomenclature . 1.15.3 Classification of nonconformities Hazardous nonconformities (defec
12、ts) Curtailment of inspection . Disposal of unacceptable lots Operating characteristic curves . Double sampling Multiple sampling . Sequential sampling . Skip-lot sampling . Comparison of single. double. multiple. and sequential sampling . 1.25.2 Average sample size Drawing of samples (random method
13、) . 1.6.3 Movinglo ts . 1.6.4 1.7.1 Description . mocessaverage . Limitingquality . Reducedinspection Singlesampling . 1.25.1 Equivalentplans . 1 1 2 2 2 3 3 3 3 4 4 4 5 5 5 5 5 5 5 5 6 6 7 7 7 7 8 9 10 10 12 12 12 13 14 14 15 16 17 17 17 18 18 19 21 . 111 ASQC S2 95 = 0759506 0001872 590 ANSVASQC S
14、2-1995 1.27 Sampling plans. schemes. and systems 22 1.28 Distributional characteristics: binomial. Poisson. and hypergeometric . 25 Section 2: The ANSUASQC 21.4 System 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 2.10 2.11 2.12 2.13 2.14 2.15 2.16 2.17 2.18 2.19 2.20 2.21 Description of ANSVASQC 21.4. 26 Pre
15、paring a specification for use in conjunction with ANSUASQC 21.4. . 27 27 Lots 29 Meaningofinspectionlevel . 31 Setting an inspection level . 32 Preferred AQLs . 35 Classification of nonconformities and nonconforming items . Nonpreferred AQLs 35 SettinganAQL . 37 Normalinspection . 39 Switching rule
16、s-example 43 Drawing a sampling plan from the tables . 38 Tightened inspection . 41 Methods for reducing the sampling risks . 44 Reduced inspection 46 Double and multiple sampling . 48 Limiting quality and the isolated lot . 50 Sample sizes . 50 Operating characteristic curves . 51 The AOQL table (s
17、ee 1.13 for theory) . 51 Nomograms 53 iV ASQC S2 95 m 0759506 OOOLB73 427 m Foreword This guide to the practice of sampling procedures for inspection by attributes is written to assist in the proper applica- tion of the standard ANSVASQC Z1.4. Much of what is included in this guide applies equally w
18、ell to other standards for acceptance of lots by sampling procedures. There are many other standards, mostly in foreign countries, based upon the same practice as that set forth in 21.4. In fact, the system of selecting sample sizes and acceptance numbers based on the lot size and the choice of an A
19、QL was based on the MIL-STD-105D tables. MIL-STD-lO5E uses the same tables and so does the International Organization for Standardization (ISO) in IS0 2859-1, as do most of the national standards. The operating characteristics of 21.4 are matched closely by the operating Characteristics of ANSVASQC
20、Z1.9, which establishes a procedure for the acceptance of lots on the basis of fraction nonconforming by variables. This is similar I I l I to the well-known MIL-STD-414, but 414 has not been updated to match the operating characteristics of 105D. It still matches the operating characteristics of MI
21、L-STD- 105A, which has somewhat different sampling pians. IS0 Technical Committee 69 on Application of Statistical Methods has a series of standards that are quite similar to those of the U.S. military standards and the ANSVASQC standards. These are written in British English, whereas American Engli
22、sh is used here. Below is a list of equivalent standards which those dealing in the international arena may encounter. The operating characteristics are similar. Occasionally some seem quite different, as will be pointed out when we list the variable sampling plans for percent nonconforming. Some of
23、 these standards follow: ANSVASQC 21.4 = IS0 2859-1 = MIL-STD-105E, Sampling Procedures and Tables for Inspection by Attributes. ANSVASQC Z1.9 = IS0 3951, Sampling Procedures and Tables for Inspection by Variables for Percent Noncon- forming, which uses a graphical approach in applying the acceptanc
24、e criteria. Both have operating characteristics essentially the same as the attribute plans above. MIL-STD-414 DOES NOT MATCH ANY OF THE ABOVE! ANSVASQC Q3k IS0 2859-2, Sampling Procedures and Tables for Inspection of Isolated Lots by Attributes. ANSVASQC S1= IS0 2859-3, An Attribute Skip-Lot Sampli
25、ng Program. The symbol “=“ is indicative of the standards being equivalent. V ASQC S2 95 0759506 0001874 363 = ANSVASQC S2- 1995 SECTION 1: GENERAL INTRODUCTION TO ACCEPTANCE SAMPLING 1.0 Introduction This general introduction to sampling inspection is pro- vided to describe the attribute sampling s
26、chemes set forth in ANSVASQC Z1.4, ANSVASQC Q3, ANSVASQC S3, and in the similar plans of the International Organization for Standardization (ISO) and in MIL-STD- 105. This in- troduction treats the subject of sampling inspection by at- tributes in a general way, introduces the essential operating pr
27、ocedures, and the ways the schemes were de- signed to be used. To fully understand the concepts and their application it would be helpful for readers to have: ANSVASQC Z1.4 Sampling procedures for in- spection by attributes- Sampling plans indexed by ac- ceptable quality level (AQL) for lot-by-lot i
28、nspection ANSVASQC Q3 ANSVASQC S 1 IS0 8422 IS0 TR8550 Sampling procedures for in- spection by attributes- Sampling plans indexed by limiting qualis, (LQ) for iso- lated lot inspection Sampling procedures for in- spection by attributes- Skip-lot sampling procedures Sequential sampling plans for insp
29、ection by attributes Guide for the selection of an acceptance sampling system, scheme orplan for inspection of discrete items in lots. The individual Parts of the standards extend this intro- ductory explanation to more specific uses of the proce- dures which are appropriate for the particular Part
30、or Standard. It is emphasized that ANSUASQC Z1.4 provides sam- pling schemes indexed by AQL. The quality measure used can be percent nonconforming or the number of nonconformities per hundred items. ANSVASQC Z1.4 was developed for the inspection of a continuing series of lots all originating from th
31、e same source, since in this situation adequate protection is possible by use of the switching rules, i.e., switching from normal to tight- ened inspection, should a certain (limiting) number of unacceptable batches be found in a short series of suc- cessive batches. If switching is indicated, it is
32、 wise to question the immediately preceding batches if they are available. ANSVASQC 43 provides sampling plans arranged for use when individual or isolated lots are to be sampled. These sampling plans are in many instances identical with those in ANSVASQC 21.4. All the tables of sampling plans in AN
33、SVASQC Q3 include information regarding the quality level required to assure a high probability of lot acceptance. It is recommended that ANSVASQC Q3 be used for individual or isolated lots rather than ANSVASQC Z1.4. This document contains quality lev- els obtained by use of the hypergeometric proce
34、dure. ANSVASQC S1 provides skip-lot procedures for use when the process quality is markedly superior to the AQL for a defined long period of delivery or observa- tion. When the quality level is in this state of excellence, it is sometimes more economical to use ANSVASQC S 1 than to use the reduced s
35、ampling procedure of ANSV ASQC Z1.4. ANSVASQC Z1.4 and ANSVASQC S 1 are applicable to a continuing series of lots from a single source. IS0 8422 provides a method of establishing sequential sampling plans of essentially equivalent discriminatory power to individual plans of ANSVASQC 21.4 and ANSVASQ
36、C Q3. A complementary system of sampling plans by variables also indexed by AQL, is provided by ANSVASQC Z1.9, Sampling procedure and tables for inspection by vari- ables for percent nonconforming, and by IS0 8423, Se- quential sampling plans for inspection by variables for percent nonconforming (kn
37、own standard deviation). In many situations producers have established process controls which customers have accepted as proof of qual- ity and have therefore suspended sampling inspection on a regular basis. 1.1 Scope Section 1 gives general information on methods of ac- ceptance sampling inspectio
38、n with particular reference to the sampling procedures and tables for inspection by 1 ASQC S2 95 w 075950b OOOL875 2TT ANSVASQC S2-1995 attributes given in ANSVASQC 21.4, ANSIASQC 43, ANSVASQC S 1, and IS0 8422. This document explains the terms used, describes the various schemes and plans, gives pr
39、actical advice on sampling inspection, and dis- cusses some of the theoretical aspects. Section 2 extends the introduction of acceptance sam- pling of Section 1 and amplifies the introductory text and instructions contained in ANSIASQC Z1.4, by giving detailed comments and examples to assist in usin
40、g the method of sampling inspection that constitutes the ANSIASQC Z1.4 sampling system. 1.2 Objective of sampling inspection The primary objective of sampling inspection is to en- courage the producer to provide a level of quality equal to or better than a contractually agreed level, and by the samp
41、ling procedure ensure that the customer receives a quality that is equal to or better than that which was spec- ified and agreed to by contract. Actually what the cus- tomer desires is perfect quality, shipments with no nonconforming items. There is a tendency to divert the responsibility for qual-
42、ity from the producer to the inspector. This may happen when production views inspection as being there to sort out nonconforming product so that, within limits, pro- duction errors will be caught by inspection. When non- conforming product gets to the customer, production people then view it as the
43、 inspectors fault rather than their own. This tendency must be avoided. Lot-by-lot inspection can also provide evidence that the efforts to continuously improve quality, a modern objec- tive, are successful. Acceptance sampling should not be thought of as a sub- stitute for process controls which ca
44、n achieve very low levels (parts per million) of nonconforming items. 1.3 Acceptance sampling Acceptance sampling inspection has the ment of putting the responsibility for quality where it belongs-with the producer. The producer must see that the quality of the product is right, otherwise there will
45、 be much trouble and expense with unacceptable lots. Sampling inspection can and should lead to less inspection work, lower cost, and good quality for the customer. To accomplish this, acceptance inspection results must be viewed from two viewpoints: one, as a gate through which only acceptable prod
46、uct will be allowed to pass, and two, as creating a learning situation where informa- tion gathered about the product is fed back to production so that future product can be improved in quality. The sampling inspection schemes of the referenced stan- dards provide for quantification of the risks of
47、accepting unsatisfactory product, the risks of not accepting satis- factory product, and for choosing a plan that allows no more risk than is acceptable. In addition to the ANSIASQC sampling plans listed in 1.0 which are based on well-defined probability func- tions, there are several other practice
48、s: a) sampling based on experience with the product, the process, the supplier, and the consumer (see 1.3.1); b) ad hoc sampling, for example, the inspection of a fixed percentage, or occasional random checks (see 1.3.2); c) 100% inspection (see 1.3.3); d) other sampling practices (see 1.3.4). 1.3.1
49、 Statistical sampling Sampling based on experience with the product, the process, the supplier, and the consumer can be statisti- cally evaluated and, based on the results obtained, future sampling can be planned. An example is the procedure set forth in ANSIASQC 21.4 which uses a set of switching rules. When quality is very good, it is possible to go to reduced inspection. The procedure set forth in ANWASQC Z1.4 gives rules for switching among normal, tightened, or reduced in- spection based on statistical evaluation of the product re- ceived at inspection. When quality is signific
