1、raising standards worldwide NO COPYING WITHOUT BSI PERMISSION EXCEPT AS PERMITTED BY COPYRIGHT LAW BSI Standards Publication BS ISO 7870-2:2013 Control charts Part 2: Shewhart control chartsBS ISO 7870-2:2013 BRITISH STANDARD National foreword This British Standard is the UK implementation of ISO 78
2、70-2:2013. It supersedes BS 7785:1994, which is withdrawn. The UK participation in its preparation was entrusted to T e c h n i c a l C o m m i t t e e S S / 4 , S t a t i s t i c a l P r o c e s s M a n a g e m e n t . A list of organizations represented on this committee can be obtained on request
3、 to its secretary. This publication does not purport to include all the necessary provisions of a contract. Users are responsible for its correct application. The British Standards Institution 2013. Published by BSI Standards Limited 2013. ISBN 978 0 580 68960 4 ICS 03.120.30 Compliance with a Briti
4、sh Standard cannot confer immunity from legal obligations. This British Standard was published under the authority of the Standards Policy and Strategy Committee on 30 April 2013. Amendments issued since publication Date T e x t a f f e c t e dBS ISO 7870-2:2013 ISO 2013 Control charts Part 2: Shewh
5、art control charts Cartes de contrle Partie 2: Cartes de contrle de Shewhart INTERNATIONAL STANDARD ISO 7870-2 First edition 2013-04-01 Reference number ISO 7870-2:2013(E)BS ISO 7870-2:2013ISO 7870-2:2013(E)ii ISO 2013 All rights reserved COPYRIGHT PROTECTED DOCUMENT ISO 2013 All rights reserved. Un
6、less otherwise specified, no part of this publication may be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the
7、 address below or ISOs member body in the country of the requester. ISO copyright office Case postale 56 CH-1211 Geneva 20 Tel. + 41 22 749 01 11 Fax + 41 22 749 09 47 E-mail copyrightiso.org Web www.iso.org Published in SwitzerlandBS ISO 7870-2:2013ISO 7870-2:2013(E) ISO 2013 All rights reserved ii
8、i Contents Page Foreword iv Introduction v 1 Scope . 1 2 Normative references 1 3 T erms, definitions and s ymbols 1 3.1 General . 1 3.2 Symbols . 1 4 Nature of Shewhart control charts . 3 5 T ypes of c ontr ol charts. 5 5.1 Control charts where no pre-specified values are given . 5 5.2 Control char
9、ts with respect to given pre-specified values 6 5.3 Types of variables and attributes control charts . 6 6 V ariables c ontr ol charts 7 6.1 Mean ( X ) chart and range (R) chart or mean ( X ) chart and standard deviation (s) chart 8 6.2 Control chart for individuals (X) and control chart for moving
10、ranges (R m ) 9 6.3 Control charts for medians ( X )10 7 C ontr ol pr oc edur e and int erpr etation for v ariables c ontr ol charts 11 7.1 Collect preliminary data 11 7.2 Examine the s (or R) chart 11 7.3 Remove assignable causes and revise the chart . .11 7.4 Examine the X chart .12 7.5 Ongoing mo
11、nitoring of process .12 8 P att ern t ests for assignable causes of v ariation .12 9 Pr oc ess c ontr ol, pr oc ess capability , and pr oc ess impr o v ement.13 10 A ttribut es c ontr ol charts .15 11 Pr eliminary c onsider ations befor e starting a c ontr ol chart 17 11.1 Choice of critical to qual
12、ity (CTQ) characteristics describing the process to control .17 11.2 Analysis of the process 17 11.3 Choice of rational subgroups .17 11.4 Frequency and size of subgroups . 18 11.5 Preliminary data collection .18 11.6 Out of control action plan .18 12 Steps in the construction of control charts 18 1
13、2.1 Determine data collection strategy 19 12.2 Data collection and computation .20 12.3 Plotting X chart and R chart .20 13 Caution with Shewhart control charts 20 13.1 General caution 21 13.2 Correlated data 22 13.3 Use of alternative rule to the three-sigma rule 22 Annex A (informative) Illustrati
14、ve examples 24 Annex B (informative) Pr actical notic es on the patt ern t ests for assignable causes of v ariation .42 Bibliogr aph y .44BS ISO 7870-2:2013ISO 7870-2:2013(E) Foreword ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies (ISO
15、 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 that committee. International organizations, gover
16、nmental 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 accordance with the rules given in the ISO/IEC D
17、irectives, 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 requires approval by at least 75 % of the member
18、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 7870-2 was prepared by Technical Committee ISO/TC 69, Applications of stat
19、istical methods, Subcommittee SC 4, Applications of statistical methods in process management. This first edition cancels and replaces ISO 8258:1991, which has been technically revised. ISO 7870 consists of the following parts, under the general title Control charts: Part 1: General guidelines Part
20、2: Shewhart control charts Part 3: Acceptance control charts Part 4: Cumulative sum charts Part 5: Specialized control charts EWMA control charts will from the subject of a future Part 6.iv ISO 2013 All rights reservedBS ISO 7870-2:2013ISO 7870-2:2013(E) Introduction A traditional approach to manufa
21、cturing has been to depend on production to make the product and on quality control to inspect the final product and screen out items not meeting specifications. This strategy of detection is often wasteful and uneconomical because it involves after-the-event inspection when the wasteful production
22、has already occurred. Instead, it is much more effective to institute a strategy of prevention to avoid waste by not producing unusable output in the first place. This can be accomplished by gathering process information and analysing it so that timely action can be taken on the process itself. Dr.
23、Walter Shewhart in 1924 proposed the control chart as a graphical means of applying the statistical principles of significance to the control of a process. Control chart theory recognizes two kinds of variability. The first kind is random variability due to “chance causes” (also known as “common/nat
24、ural/ random/inherent/uncontrollable causes”). This is due to the wide variety of causes that are consistently present and not readily identifiable, each of which constitutes a very small component of the total variability but none of which contributes any significant amount. Nevertheless, the sum o
25、f the contributions of all of these unidentifiable random causes is measurable and is assumed to be inherent to the process. The elimination or correction of common causes may well require a decision to allocate resources to fundamentally change the process and system. The second kind of variability
26、 represents a real change in the process. Such a change can be attributed to some identifiable causes that are not an inherent part of the process and which can, at least theoretically, be eliminated. These identifiable causes are referred to as “assignable causes” (also known as special/unnatural/s
27、ystematic/controllable causes) of variation. They may be attributable to such matters as the lack of uniformity in material, a broken tool, workmanship or procedures, the irregular performance of equipment, or environmental changes. A process is said to be in statistical control, or simply “in contr
28、ol”, when the process variability results only from random causes. Once this level of variation is determined, any deviation from this level is assumed to be the result of assignable causes that should be identified and eliminated. Statistical process control is a methodology for establishing and ma
29、intaining a process at an acceptable and stable level so as to ensure conformity of products and services to specified requirements. The major statistical tool used to do this is the control chart, which is a graphical method of presenting and comparing information based on a sequence of observation
30、s representing the current state of a process against limits established after consideration of inherent process variability called process capability. The control chart method helps first to evaluate whether or not a process has attained, or continues in, a state of statistical control. When in suc
31、h a state the process is deemed to be stable and predictable and further analysis as to the ability of the process to satisfy the requirements of the customer can then be conducted. The control chart also can be used to provide a continuous record of a quality characteristic of the process output wh
32、ile process activity is ongoing. Control charts aid in the detection of unnatural patterns of variation in data resulting from repetitive processes and provide criteria for detecting a lack of statistical control. The use of a control chart and its careful analysis leads to a better understanding of
33、 the process and will often result in the identification of ways to make valuable improvements. ISO 2013 All rights reserved vBS ISO 7870-2:2013BS ISO 7870-2:2013Control charts Part 2: Shewhart control charts 1 Scope This International Standard establishes a guide to the use and understanding of the
34、 Shewhart control chart approach to the methods for statistical control of a process. This International Standard is limited to the treatment of statistical process control methods using only the Shewhart system of charts. Some supplementary material that is consistent with the Shewhart approach, su
35、ch as the use of warning limits, analysis of trend patterns and process capability is briefly introduced. There are, however, several other types of control chart procedures, a general description of which can be found in ISO 7870-1. 2 Normative references The following referenced documents, in whol
36、e or in part, are normatively referenced in this document and are indispensable for its application. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. ISO 3534-2, Statistics Vocabulary and s
37、ymbols Part 2: Applied statistics ISO 16269-4, Statistical interpretation of data Part 4: Detection and treatment of outliers ISO 5479, Statistical interpretation of data Tests for departure from the normal distribution ISO 22514 (all parts), Statistical methods in process management Capability and
38、performance 3 T erms, d efinitions and s ymbols 3.1 General For the purposes of this document, the terms and definitions given in ISO 3534-2:2006 apply. 3.2 S ymbols NOTE The ISO/IEC Directives makes it necessary to depart from common SPC usage in respect to the differentiation between abbreviated t
39、erms and symbols. In ISO standards an abbreviated term and its symbol can differ in appearance in two ways: by font and by layout. To distinguish between abbreviated terms and symbols, abbreviated terms are given in Cambria upright and symbols in Cambria or Greek italics, as applicable. Whereas abbr
40、eviated terms can contain multiple letters, symbols consist only of a single letter. For example, the conventional abbreviation of upper control limit, UCL, is valid but its symbol in equations becomes U CL . The reason for this is to avoid misinterpretation of compound letters as an indication of m
41、ultiplication. In cases of long established practice where a symbol and/or abbreviated term means different things in different applications, it is necessary to use a field limiter, thus , to distinguish between them. This avoids the alienation of practitioners by the creation of unfamiliar abbrevia
42、ted terms and symbols in their particular field that are unlike all related texts, operational manuals and dedicated software programs. An example is the abbreviated term R and symbol R which means different things in metrology from that in acceptance sampling and statistical process control. The ab
43、breviated term R is differentiated thus: INTERNATIONAL ST ANDARD ISO 7870-2:2013(E) ISO 2013 All rights reserved 1BS ISO 7870-2:2013ISO 7870-2:2013(E) R metrology reproducibility limit R SPC and acceptance sampling range For the purposes of this document, the following symbols apply. n Subgroup size
44、; the number of sample observations per subgroup k Number of subgroups L Lower specification limit L CL Lower control limit U Upper specification limit U CL Upper control limit X Measured quality characteristic (individual values are expressed as (X 1 , X 2 , X 3 ,.). Sometimes the symbol Y is used
45、instead of X X (X bar) Subgroup average X (X double bar) Average of the subgroup averages True process mean value True process standard deviation value 0 A given value of X Median of a subgroup X Average of the subgroup medians R Subgroup range: difference between the largest observation and smalles
46、t observation of a subgroup R Average of the R values for all subgroups R m Moving range: the absolute value of the difference between two successive values |X 1 X 2 |,|X 2 X 3 |, etc. R m Average of the (n 1) R mvalues in a set of n observed values s Sample standard deviation obtained from values w
47、ithin a subgroup: s Average of the subgroup sample standard deviations Estimated process standard deviation value p Proportion or fraction of units in a subgroup with a given classification p Average value of the proportion or fraction np Number of units with a given classification in a subgroup2 IS
48、O 2013 All rights reservedBS ISO 7870-2:2013ISO 7870-2:2013(E) p 0 A given value of p np 0 A given value of np (for a given p 0 ) c Number of incidences in a subgroup c 0 A given value of c c Average value of the c values for all subgroups u Number of incidences per unit in a subgroup u Average valu
49、e of the u values u 0 A given value of u 4 Nature of Shewhart control charts A Shewhart control chart is a graph that is used to display a statistical measure obtained from either variables or attribute data. The control chart requires data from rational subgroups to be taken at approximately regular intervals from the process. The intervals may be defined in terms of time (for example hourly) or quantity (every lot). Usually, the data are obtained from the process in the fo
