SIST-TP ISO/TR 8550-2:2010
Guidance on the selection and usage of acceptance sampling systems for inspection of discrete items in lots -- Part 2: Sampling by attributes
Guidance on the selection and usage of acceptance sampling systems for inspection of discrete items in lots -- Part 2: Sampling by attributes
The guidance given in this part of ISO/TR 8550 is confined to acceptance sampling of products that are supplied in lots and that can be classified as consisting of discrete items (i.e. discrete articles of product). Each item in a lot can be identified and segregated from the other items in the lot and has an equal chance of being included in the sample. Each item of product is countable and has specific characteristics that are measurable or classifiable as being conforming or nonconforming (to a given specification). Standards on acceptance sampling by attributes are applicable to a wide variety of inspection situations. These include, but are not limited to, the following: a) end items, such as complete products or sub-assemblies; b) components and raw materials; c) services; d) materials in process; e) supplies in storage; f) maintenance operations; g) data or records; h) administrative procedures. Although this part of ISO/TR 8550 is written principally in terms of manufacture and production, it is applicable to the selection of sampling systems, schemes and plans for all types of product and processes as defined in ISO 9000.
Lignes directrices pour la sélection d'un système, d'un programme ou d'un plan d'échantillonnage pour acceptation pour le contrôle d'unités discrètes en lots -- Partie 2: Échantillonnage par attributs
Napotek za izbiro in uporabo sistemov prevzemnega vzorčenja za kontrolo diskretnih primerkov v partijah (lotih) - 2. del: Vzorčenje po opisnih spremenljivkah
Napotek, podan v tem delu ISO/TR 8550, je omejen na prevzemno vzorčenje produktov, ki se dobavljajo v partijah, in ki se lahko klasificirajo kot sestavljeni iz diskretnih primerkov (tj. diskretni predmeti produkta). Vsak primerek iz partije se lahko prepozna in loči od drugih primerkov iz partije ter ima enako možnost vključitve v vzorec. Vsak primerek produkta je števen in ima določene karakteristike, ki se lahko merijo in razvrstijo kot skladne ali neskladne (s podano specifikacijo). Standardi o prevzemnem vzorčenju po opisnih spremenljivkah veljajo za široko raznolikost kontrolnih situacij. Te vključujejo, vendar niso omejene na naslednje: a) končni primerki, kot so popolni proizvodi ali sestavni deli; b) komponente in surovine; c) storitve; d) materiali in procesi; e) zaloge in skladiščenje; f) vzdrževalne operacije; g) podatki ali zapisi; h) administrativni postopki. Čeprav je ta del ISO/TR 8550 napisan predvsem na ravni proizvajanja in proizvodnje, velja za izbiro vzorčnih sistemov in načrtov za vse tipe produkta in procese, kot so opredeljeni v ISO 9000.
General Information
Buy Standard
Standards Content (Sample)
SLOVENSKI STANDARD
SIST-TP ISO/TR 8550-2:2010
01-julij-2010
1DSRWHN]DL]ELURLQXSRUDERVLVWHPRYSUHY]HPQHJDY]RUþHQMD]DNRQWUROR
GLVNUHWQLKSULPHUNRYYSDUWLMDK ORWLK GHO 9]RUþHQMHSRRSLVQLKVSUHPHQOMLYNDK
Guidance on the selection and usage of acceptance sampling systems for inspection of
discrete items in lots -- Part 2: Sampling by attributes
Lignes directrices pour la sélection d'un système, d'un programme ou d'un plan
d'échantillonnage pour acceptation pour le contrôle d'unités discrètes en lots -- Partie 2:
Échantillonnage par attributs
Ta slovenski standard je istoveten z: ISO/TR 8550-2:2007
ICS:
03.120.30 8SRUDEDVWDWLVWLþQLKPHWRG Application of statistical
methods
SIST-TP ISO/TR 8550-2:2010 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
---------------------- Page: 1 ----------------------
SIST-TP ISO/TR 8550-2:2010
---------------------- Page: 2 ----------------------
SIST-TP ISO/TR 8550-2:2010
TECHNICAL ISO/TR
REPORT 8550-2
First edition
2007-10-15
Guidance on the selection and usage
of acceptance sampling systems
for inspection of discrete items in lots —
Part 2:
Sampling by attributes
Lignes directrices pour la sélection d'un système, d'un programme
ou d'un plan d'échantillonnage pour acceptation pour le contrôle
d'unités discrètes en lots —
Partie 2: Échantillonnage par attributs
Reference number
ISO/TR 8550-2:2007(E)
©
ISO 2007
---------------------- Page: 3 ----------------------
SIST-TP ISO/TR 8550-2:2010
ISO/TR 8550-2:2007(E)
PDF disclaimer
This PDF file may contain embedded typefaces. In accordance with Adobe's licensing policy, this file may be printed or viewed but
shall not be edited unless the typefaces which are embedded are licensed to and installed on the computer performing the editing. In
downloading this file, parties accept therein the responsibility of not infringing Adobe's licensing policy. The ISO Central Secretariat
accepts no liability in this area.
Adobe is a trademark of Adobe Systems Incorporated.
Details of the software products used to create this PDF file can be found in the General Info relative to the file; the PDF-creation
parameters were optimized for printing. Every care has been taken to ensure that the file is suitable for use by ISO member bodies. In
the unlikely event that a problem relating to it is found, please inform the Central Secretariat at the address given below.
COPYRIGHT PROTECTED DOCUMENT
© ISO 2007
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means,
electronic or mechanical, including photocopying and microfilm, without permission in writing from either ISO at the address below or
ISO's 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 copyright@iso.org
Web www.iso.org
Published in Switzerland
ii © ISO 2007 – All rights reserved
---------------------- Page: 4 ----------------------
SIST-TP ISO/TR 8550-2:2010
ISO/TR 8550-2:2007(E)
Contents Page
Foreword. iv
Introduction . v
1 Scope .1
2 Normative references .1
3 Selection process .2
4 International Standards for acceptance sampling of lots by attributes.2
5 Effect on the selection process of market and production conditions.6
Bibliography .14
© ISO 2007 – All rights reserved iii
---------------------- Page: 5 ----------------------
SIST-TP ISO/TR 8550-2:2010
ISO/TR 8550-2:2007(E)
Foreword
ISO (the International 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
been 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 standardization.
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 for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
In exceptional circumstances, when a technical committee has collected data of a different kind from that
which is normally published as an International Standard (“state of the art”, for example), it may decide by a
simple majority vote of its participating members to publish a Technical Report. A Technical Report is entirely
informative in nature and does not have to be reviewed until the data it provides are considered to be no
longer valid or useful.
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/TR 8550-2 was prepared by Technical Committee ISO/TC 69, Applications of statistical methods,
Subcommittee SC 5, Acceptance sampling.
This first edition of ISO/TR 8550-2, together with ISO/TR 8550-1 and ISO/TR 8550-3, cancels and replaces
ISO/TR 8550:1994.
ISO/TR 8550 consists of the following parts, under the general title Guidance on the selection and usage of
acceptance sampling systems for inspection of discrete items in lots:
⎯ Part 1: Acceptance sampling
⎯ Part 2: Sampling by attributes
⎯ Part 3: Sampling by variables
iv © ISO 2007 – All rights reserved
---------------------- Page: 6 ----------------------
SIST-TP ISO/TR 8550-2:2010
ISO/TR 8550-2:2007(E)
Introduction
This part of ISO/TR 8550 gives guidance on the selection of a generic acceptance sampling system, scheme
or plan for inspection by attributes from those developed by ISO/TC 69. It does this principally by reviewing
the available systems specified by various standards and by showing ways in which these can be compared
to assess their suitability for an intended application. It is assumed that the choice has already been made to
use sampling by attributes in preference to sampling by variables.
A corresponding guidance document on the selection of a generic acceptance sampling system, scheme or
plan for inspection by variables is given in ISO/TR 8550-3.
© ISO 2007 – All rights reserved v
---------------------- Page: 7 ----------------------
SIST-TP ISO/TR 8550-2:2010
---------------------- Page: 8 ----------------------
SIST-TP ISO/TR 8550-2:2010
TECHNICAL REPORT ISO/TR 8550-2:2007(E)
Guidance on the selection and usage of acceptance sampling
systems for inspection of discrete items in lots —
Part 2:
Sampling by attributes
1 Scope
The guidance given in this part of ISO/TR 8550 is confined to acceptance sampling of products that are
supplied in lots and that can be classified as consisting of discrete items (i.e. discrete articles of product).
Each item in a lot can be identified and segregated from the other items in the lot and has an equal chance of
being included in the sample. Each item of product is countable and has specific characteristics that are
measurable or classifiable as being conforming or nonconforming (to a given specification).
Standards on acceptance sampling by attributes are applicable to a wide variety of inspection situations.
These include, but are not limited to, the following:
a) end items, such as complete products or sub-assemblies;
b) components and raw materials;
c) services;
d) materials in process;
e) supplies in storage;
f) maintenance operations;
g) data or records;
h) administrative procedures.
Although this part of ISO/TR 8550 is written principally in terms of manufacture and production, it is applicable
to the selection of sampling systems, schemes and plans for all types of product and processes as defined in
ISO 9000.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition listed applies. For undated references, the latest edition of the referenced
document (including any amendment) applies.
ISO/TR 8550-1:2007, Guidance on the selection and usage of acceptance sampling systems for inspection of
discrete items in lots — Part 1: Acceptance sampling
© ISO 2007 – All rights reserved 1
---------------------- Page: 9 ----------------------
SIST-TP ISO/TR 8550-2:2010
ISO/TR 8550-2:2007(E)
3 Selection process
The task of selecting a suitable sampling system, scheme or plan is influenced by production and marketing
conditions. In addition, the economics of the sampling system, the resources of the inspection organization
and other aspects need to be considered. Therefore, the selection process becomes complex and rarely is
there one method of acceptance sampling that fits all situations even though they might appear to be similar.
Tables 1, 2 and 3, together with Figures 1 and 2, illustrate the process of selecting a sampling system,
scheme or plan. The tables provide “candidate” sampling systems, schemes and plans to fit given inspection
situations, production conditions and market conditions, respectively. The method suggested is to review
Tables 1, 2 and 3 and to select as many “candidate plans” as fit the situation. These candidates should then
be reviewed through Figure 1 or 2 so that the user finally arrives at a system, scheme or plan that is most
feasible and economical for the situation.
The initial selection decisions are for long and short production runs, isolated lots, and attribute or variables
sampling; these decisions will point to one or more possible sampling schemes specified in standards.
Figures 1 and 2 are not flow charts. They show, in summary form, the procedures contained within each
standard for arriving at a sampling system, scheme or plan. The summaries are listed side by side, which
allows direct comparison between them. The solid connecting lines indicate the primary course to be followed
in choosing and operating a standard; the broken lines indicate alternatives. These alternatives are applicable
only under certain conditions. Text references and notes to the figures are given for guidance in following
these procedures. The standards and procedures given below the horizontal line in each of the two figures are
for use when economy of average sample size is an important consideration.
The procedure presented in Figure 1 is applicable when production is continuous and there are more than
10 lots of product undergoing inspection. International Standards from the ISO 2859 series are included here
as “candidate’ systems”.
Figure 2 is applicable when other conditions prevail, e.g. when there is no continuity of production, when the
number of lots is 10 or fewer, when the product warranty does not survive acceptance and/or when the
presence of a small number of nonconforming items can result in a large loss.
A final selection should be based on both the requirements of the situation and the resources of the inspection
organization.
When selecting a sampling system, scheme or plan, it may become apparent that it has one or more
deficiencies. The cases in which only one method is appropriate are few. Iterative investigations usually
identify two or more methods. The most economical and appropriate method should be chosen.
4 International Standards for acceptance sampling of lots by attributes
4.1 General
This clause summarizes the salient features of each of the current standards concerned with acceptance
sampling methods by attributes. The summaries of the scope and application of the available standards
should enable a user to select those standards which are most likely to suit a given purpose.
The comparisons between the various standard acceptance sampling systems in these summaries are not
enough to allow a final selection of a sampling system, scheme or plan to be made in a particular situation.
Before this can be done, a number of factors need to be understood and considered. These factors are
reviewed in Clause 5.
2 © ISO 2007 – All rights reserved
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SIST-TP ISO/TR 8550-2:2010
ISO/TR 8550-2:2007(E)
4.2 ISO 2859−1: Sampling procedures for inspection by attributes — Part 1: Sampling
schemes indexed by acceptance quality limit (AQL) for lot-by-lot inspection
This presents a sampling system indexed by lot-size ranges, inspection levels and AQLs, and specifies
sampling plans and procedures for inspection by attributes of discrete items. It contains sampling plans for
single, double and multiple sampling indexed by percent nonconforming and nonconformities per 100 items.
The sampling system specified in ISO 2859-1 is intended to employ tightened, normal and reduced inspection
on a continuing series of lots to achieve customer protection while assuring the producer that, if quality is
better than the AQL, acceptance will occur most of the time.
The objective is to maintain a process average quality at least as good as the AQL that has been agreed
between the producer and the consumer, while at the same time providing an upper limit for the risk to the
consumer of accepting the occasional lot of poor quality.
The sampling plans in ISO 2859-1 may also be used for the inspection of lots in isolation, but in this case the
user is strongly advised to consult the operating characteristic curves (in Table 12 of ISO 2859-1:1999) to find
a plan which will yield the desired protection. A much simpler procedure to follow in this type of situation is
presented in ISO 2859-2.
4.3 ISO 2859-2: Sampling procedures for inspection by attributes — Part 2: Sampling plans
indexed by limiting quality (LQ) for isolated lot inspection
This establishes sampling plans indexed by limiting quality (LQ) and procedures for inspection that can be
used when the switching rules of ISO 2859-1 cannot be applied. The LQ is used to indicate the customer
protection. Procedure A is used for single lots and procedure B for lots isolated from a continuing series. Both
procedures treat the LQ as an indicator of the actual percentage nonconforming in the lots submitted, though
they can also be used in cases where quality is expressed in non-conformities per 100 items.
Procedure A is used when both the supplier and the customer wish to regard the lot in isolation, but it is also
used as the default procedure where there is no specific instruction to use procedure B.
Procedure B is used when the supplier regards the lot as one of a continuing series, but the customer
considers the lot received in isolation. The sampling plans employed permit a producer to maintain consistent
procedures for customers, irrespective of whether the customers receive individual lots or a continuing series
of lots. The manufacturer is concerned with all of the production but the individual customer only with the
particular lot received.
For procedure A, sampling plans are identified by their lot size and LQ; for procedure B, they are identified by
lot size, LQ and inspection level.
Procedure A includes sampling plans with an acceptance number of zero, but these are excluded from
procedure B. Double and multiple sampling plans can be used as alternatives to single sampling plans in
procedure B and for the non-zero acceptance number plans in procedure A.
4.4 ISO 2859-3: Sampling procedures for inspection by attributes — Part 3: Skip-lot
sampling procedures
This specifies a sampling system that extends the procedures specified in ISO 2859-1. It provides generic
attribute skip-lot sampling procedures, for reducing the inspection effort on products submitted by suppliers
who have demonstrated their ability to control, in an effective manner, all facets of product quality and to
produce superior quality material consistently. However, the standard proscribes these procedures for the
inspection of product characteristics that involve the safety of personnel.
The skip-lot programme uses the acceptance sampling plans specified in ISO 2859-1 and is intended only for
a continuing series of lots; it is not appropriate for isolated lots. All lots in a series are expected to be of a
similar quality and there should be no reason to believe that lots that have not been inspected are of a poorer
quality than those that have.
© ISO 2007 – All rights reserved 3
---------------------- Page: 11 ----------------------
SIST-TP ISO/TR 8550-2:2010
ISO/TR 8550-2:2007(E)
In a skip-lot sampling procedure, some lots in a series are accepted without inspection when the sampling
results for a stated number of immediately preceding lots meet criteria that are provided in the standard. The
lots to be inspected are chosen randomly with a stated frequency, called the “skip-lot frequency”.
4.5 ISO 2859-4: Sampling procedures for inspection by attributes — Part 4: Procedures for
assessment of declared quality levels
This provides sampling plans and procedures for assessing whether the quality level of a lot or process
conforms to a declared value. The sampling plans have been devised to have a risk of less than 5 % of
contradicting a correct declared quality level (DQL). Conversely, the risk is 10 % of failing to contradict an
incorrect DQL, which is related to the limiting quality ratio. ISO 2859-4 provides sampling plans corresponding
to three levels of discriminatory ability.
In contrast to other parts of ISO 2859, the procedures in ISO 2859-4 are not intended for the acceptance
assessment of lots. In general, the balancing of the risks of reaching incorrect conclusions for assessment
procedures differs from the balancing in the procedures for acceptance sampling.
ISO 2859-4 may be used for various forms of quality inspection in situations where objective evidence of
conformity to some DQL is to be provided by means of inspection of a sample. The procedures are applicable
to entities such as lots, process output, etc., that allow random samples of individual items to be selected.
ISO 2859-4 is primarily intended to be used when the quantity of interest is the number or fraction of
nonconforming items, but may also be used as an approximation when the quantity of interest is the number
of nonconformities or the number of nonconformities per item.
4.6 ISO 2859-5: Sampling procedures for inspection by attributes — Part 5: System of
sequential sampling plans indexed by acceptance quality limit (AQL) for lot-by-lot inspection
ISO 2859-5 presents a sampling system of sequential sampling plans for lot-by-lot inspection that
supplements and matches the ISO 2859-1 system of single, double and multiple sampling plans by attributes.
Accordingly, the plans are indexed by lot size ranges, inspection levels and AQL and the system includes
switching rules. (For the relationship between AQL, LQ, CRP and PRP, see Figures 1 and 2 of
ISO/TR 8550-1:2007.)
In sequential sampling, items are selected randomly and inspected one after another, and a cumulative count
is kept of the number of nonconforming items or number of nonconformities. The decision to classify the lot as
acceptable or not acceptable can occur at almost any stage and, for sequential sampling by attributes,
depends on the number of items inspected and the cumulative number of nonconforming items or
nonconformities found up to that point.
ISO 2859-5 provides procedures, based on a sequential assessment of inspection results, that encourage a
supplier to supply lots of a good quality with a high probability of acceptance while maintaining an upper limit
for the risk of consumers accepting lots of poor quality.
4.7 ISO 2859-10: Sampling procedures for inspection by attributes — Part 10: Introduction
to the ISO 2859 series of standards for sampling for inspection by attributes
This is a companion document to this part of ISO/TR 8550 and can be read in conjunction with it, if desired,
but this is not essential. It is not a source of sampling schemes or plans.
Clause 4 of ISO 2859-10:2006 is essentially an introduction to the sampling schemes employed in ISO 2859-1,
ISO 2859-2 and ISO 28
...
TECHNICAL ISO/TR
REPORT 8550-2
First edition
2007-10-15
Guidance on the selection and usage
of acceptance sampling systems
for inspection of discrete items in lots —
Part 2:
Sampling by attributes
Lignes directrices pour la sélection d'un système, d'un programme
ou d'un plan d'échantillonnage pour acceptation pour le contrôle
d'unités discrètes en lots —
Partie 2: Échantillonnage par attributs
Reference number
ISO/TR 8550-2:2007(E)
©
ISO 2007
---------------------- Page: 1 ----------------------
ISO/TR 8550-2:2007(E)
PDF disclaimer
This PDF file may contain embedded typefaces. In accordance with Adobe's licensing policy, this file may be printed or viewed but
shall not be edited unless the typefaces which are embedded are licensed to and installed on the computer performing the editing. In
downloading this file, parties accept therein the responsibility of not infringing Adobe's licensing policy. The ISO Central Secretariat
accepts no liability in this area.
Adobe is a trademark of Adobe Systems Incorporated.
Details of the software products used to create this PDF file can be found in the General Info relative to the file; the PDF-creation
parameters were optimized for printing. Every care has been taken to ensure that the file is suitable for use by ISO member bodies. In
the unlikely event that a problem relating to it is found, please inform the Central Secretariat at the address given below.
COPYRIGHT PROTECTED DOCUMENT
© ISO 2007
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means,
electronic or mechanical, including photocopying and microfilm, without permission in writing from either ISO at the address below or
ISO's 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 copyright@iso.org
Web www.iso.org
Published in Switzerland
ii © ISO 2007 – All rights reserved
---------------------- Page: 2 ----------------------
ISO/TR 8550-2:2007(E)
Contents Page
Foreword. iv
Introduction . v
1 Scope .1
2 Normative references .1
3 Selection process .2
4 International Standards for acceptance sampling of lots by attributes.2
5 Effect on the selection process of market and production conditions.6
Bibliography .14
© ISO 2007 – All rights reserved iii
---------------------- Page: 3 ----------------------
ISO/TR 8550-2:2007(E)
Foreword
ISO (the International 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
been 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 standardization.
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 for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
In exceptional circumstances, when a technical committee has collected data of a different kind from that
which is normally published as an International Standard (“state of the art”, for example), it may decide by a
simple majority vote of its participating members to publish a Technical Report. A Technical Report is entirely
informative in nature and does not have to be reviewed until the data it provides are considered to be no
longer valid or useful.
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/TR 8550-2 was prepared by Technical Committee ISO/TC 69, Applications of statistical methods,
Subcommittee SC 5, Acceptance sampling.
This first edition of ISO/TR 8550-2, together with ISO/TR 8550-1 and ISO/TR 8550-3, cancels and replaces
ISO/TR 8550:1994.
ISO/TR 8550 consists of the following parts, under the general title Guidance on the selection and usage of
acceptance sampling systems for inspection of discrete items in lots:
⎯ Part 1: Acceptance sampling
⎯ Part 2: Sampling by attributes
⎯ Part 3: Sampling by variables
iv © ISO 2007 – All rights reserved
---------------------- Page: 4 ----------------------
ISO/TR 8550-2:2007(E)
Introduction
This part of ISO/TR 8550 gives guidance on the selection of a generic acceptance sampling system, scheme
or plan for inspection by attributes from those developed by ISO/TC 69. It does this principally by reviewing
the available systems specified by various standards and by showing ways in which these can be compared
to assess their suitability for an intended application. It is assumed that the choice has already been made to
use sampling by attributes in preference to sampling by variables.
A corresponding guidance document on the selection of a generic acceptance sampling system, scheme or
plan for inspection by variables is given in ISO/TR 8550-3.
© ISO 2007 – All rights reserved v
---------------------- Page: 5 ----------------------
TECHNICAL REPORT ISO/TR 8550-2:2007(E)
Guidance on the selection and usage of acceptance sampling
systems for inspection of discrete items in lots —
Part 2:
Sampling by attributes
1 Scope
The guidance given in this part of ISO/TR 8550 is confined to acceptance sampling of products that are
supplied in lots and that can be classified as consisting of discrete items (i.e. discrete articles of product).
Each item in a lot can be identified and segregated from the other items in the lot and has an equal chance of
being included in the sample. Each item of product is countable and has specific characteristics that are
measurable or classifiable as being conforming or nonconforming (to a given specification).
Standards on acceptance sampling by attributes are applicable to a wide variety of inspection situations.
These include, but are not limited to, the following:
a) end items, such as complete products or sub-assemblies;
b) components and raw materials;
c) services;
d) materials in process;
e) supplies in storage;
f) maintenance operations;
g) data or records;
h) administrative procedures.
Although this part of ISO/TR 8550 is written principally in terms of manufacture and production, it is applicable
to the selection of sampling systems, schemes and plans for all types of product and processes as defined in
ISO 9000.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition listed applies. For undated references, the latest edition of the referenced
document (including any amendment) applies.
ISO/TR 8550-1:2007, Guidance on the selection and usage of acceptance sampling systems for inspection of
discrete items in lots — Part 1: Acceptance sampling
© ISO 2007 – All rights reserved 1
---------------------- Page: 6 ----------------------
ISO/TR 8550-2:2007(E)
3 Selection process
The task of selecting a suitable sampling system, scheme or plan is influenced by production and marketing
conditions. In addition, the economics of the sampling system, the resources of the inspection organization
and other aspects need to be considered. Therefore, the selection process becomes complex and rarely is
there one method of acceptance sampling that fits all situations even though they might appear to be similar.
Tables 1, 2 and 3, together with Figures 1 and 2, illustrate the process of selecting a sampling system,
scheme or plan. The tables provide “candidate” sampling systems, schemes and plans to fit given inspection
situations, production conditions and market conditions, respectively. The method suggested is to review
Tables 1, 2 and 3 and to select as many “candidate plans” as fit the situation. These candidates should then
be reviewed through Figure 1 or 2 so that the user finally arrives at a system, scheme or plan that is most
feasible and economical for the situation.
The initial selection decisions are for long and short production runs, isolated lots, and attribute or variables
sampling; these decisions will point to one or more possible sampling schemes specified in standards.
Figures 1 and 2 are not flow charts. They show, in summary form, the procedures contained within each
standard for arriving at a sampling system, scheme or plan. The summaries are listed side by side, which
allows direct comparison between them. The solid connecting lines indicate the primary course to be followed
in choosing and operating a standard; the broken lines indicate alternatives. These alternatives are applicable
only under certain conditions. Text references and notes to the figures are given for guidance in following
these procedures. The standards and procedures given below the horizontal line in each of the two figures are
for use when economy of average sample size is an important consideration.
The procedure presented in Figure 1 is applicable when production is continuous and there are more than
10 lots of product undergoing inspection. International Standards from the ISO 2859 series are included here
as “candidate’ systems”.
Figure 2 is applicable when other conditions prevail, e.g. when there is no continuity of production, when the
number of lots is 10 or fewer, when the product warranty does not survive acceptance and/or when the
presence of a small number of nonconforming items can result in a large loss.
A final selection should be based on both the requirements of the situation and the resources of the inspection
organization.
When selecting a sampling system, scheme or plan, it may become apparent that it has one or more
deficiencies. The cases in which only one method is appropriate are few. Iterative investigations usually
identify two or more methods. The most economical and appropriate method should be chosen.
4 International Standards for acceptance sampling of lots by attributes
4.1 General
This clause summarizes the salient features of each of the current standards concerned with acceptance
sampling methods by attributes. The summaries of the scope and application of the available standards
should enable a user to select those standards which are most likely to suit a given purpose.
The comparisons between the various standard acceptance sampling systems in these summaries are not
enough to allow a final selection of a sampling system, scheme or plan to be made in a particular situation.
Before this can be done, a number of factors need to be understood and considered. These factors are
reviewed in Clause 5.
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ISO/TR 8550-2:2007(E)
4.2 ISO 2859−1: Sampling procedures for inspection by attributes — Part 1: Sampling
schemes indexed by acceptance quality limit (AQL) for lot-by-lot inspection
This presents a sampling system indexed by lot-size ranges, inspection levels and AQLs, and specifies
sampling plans and procedures for inspection by attributes of discrete items. It contains sampling plans for
single, double and multiple sampling indexed by percent nonconforming and nonconformities per 100 items.
The sampling system specified in ISO 2859-1 is intended to employ tightened, normal and reduced inspection
on a continuing series of lots to achieve customer protection while assuring the producer that, if quality is
better than the AQL, acceptance will occur most of the time.
The objective is to maintain a process average quality at least as good as the AQL that has been agreed
between the producer and the consumer, while at the same time providing an upper limit for the risk to the
consumer of accepting the occasional lot of poor quality.
The sampling plans in ISO 2859-1 may also be used for the inspection of lots in isolation, but in this case the
user is strongly advised to consult the operating characteristic curves (in Table 12 of ISO 2859-1:1999) to find
a plan which will yield the desired protection. A much simpler procedure to follow in this type of situation is
presented in ISO 2859-2.
4.3 ISO 2859-2: Sampling procedures for inspection by attributes — Part 2: Sampling plans
indexed by limiting quality (LQ) for isolated lot inspection
This establishes sampling plans indexed by limiting quality (LQ) and procedures for inspection that can be
used when the switching rules of ISO 2859-1 cannot be applied. The LQ is used to indicate the customer
protection. Procedure A is used for single lots and procedure B for lots isolated from a continuing series. Both
procedures treat the LQ as an indicator of the actual percentage nonconforming in the lots submitted, though
they can also be used in cases where quality is expressed in non-conformities per 100 items.
Procedure A is used when both the supplier and the customer wish to regard the lot in isolation, but it is also
used as the default procedure where there is no specific instruction to use procedure B.
Procedure B is used when the supplier regards the lot as one of a continuing series, but the customer
considers the lot received in isolation. The sampling plans employed permit a producer to maintain consistent
procedures for customers, irrespective of whether the customers receive individual lots or a continuing series
of lots. The manufacturer is concerned with all of the production but the individual customer only with the
particular lot received.
For procedure A, sampling plans are identified by their lot size and LQ; for procedure B, they are identified by
lot size, LQ and inspection level.
Procedure A includes sampling plans with an acceptance number of zero, but these are excluded from
procedure B. Double and multiple sampling plans can be used as alternatives to single sampling plans in
procedure B and for the non-zero acceptance number plans in procedure A.
4.4 ISO 2859-3: Sampling procedures for inspection by attributes — Part 3: Skip-lot
sampling procedures
This specifies a sampling system that extends the procedures specified in ISO 2859-1. It provides generic
attribute skip-lot sampling procedures, for reducing the inspection effort on products submitted by suppliers
who have demonstrated their ability to control, in an effective manner, all facets of product quality and to
produce superior quality material consistently. However, the standard proscribes these procedures for the
inspection of product characteristics that involve the safety of personnel.
The skip-lot programme uses the acceptance sampling plans specified in ISO 2859-1 and is intended only for
a continuing series of lots; it is not appropriate for isolated lots. All lots in a series are expected to be of a
similar quality and there should be no reason to believe that lots that have not been inspected are of a poorer
quality than those that have.
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ISO/TR 8550-2:2007(E)
In a skip-lot sampling procedure, some lots in a series are accepted without inspection when the sampling
results for a stated number of immediately preceding lots meet criteria that are provided in the standard. The
lots to be inspected are chosen randomly with a stated frequency, called the “skip-lot frequency”.
4.5 ISO 2859-4: Sampling procedures for inspection by attributes — Part 4: Procedures for
assessment of declared quality levels
This provides sampling plans and procedures for assessing whether the quality level of a lot or process
conforms to a declared value. The sampling plans have been devised to have a risk of less than 5 % of
contradicting a correct declared quality level (DQL). Conversely, the risk is 10 % of failing to contradict an
incorrect DQL, which is related to the limiting quality ratio. ISO 2859-4 provides sampling plans corresponding
to three levels of discriminatory ability.
In contrast to other parts of ISO 2859, the procedures in ISO 2859-4 are not intended for the acceptance
assessment of lots. In general, the balancing of the risks of reaching incorrect conclusions for assessment
procedures differs from the balancing in the procedures for acceptance sampling.
ISO 2859-4 may be used for various forms of quality inspection in situations where objective evidence of
conformity to some DQL is to be provided by means of inspection of a sample. The procedures are applicable
to entities such as lots, process output, etc., that allow random samples of individual items to be selected.
ISO 2859-4 is primarily intended to be used when the quantity of interest is the number or fraction of
nonconforming items, but may also be used as an approximation when the quantity of interest is the number
of nonconformities or the number of nonconformities per item.
4.6 ISO 2859-5: Sampling procedures for inspection by attributes — Part 5: System of
sequential sampling plans indexed by acceptance quality limit (AQL) for lot-by-lot inspection
ISO 2859-5 presents a sampling system of sequential sampling plans for lot-by-lot inspection that
supplements and matches the ISO 2859-1 system of single, double and multiple sampling plans by attributes.
Accordingly, the plans are indexed by lot size ranges, inspection levels and AQL and the system includes
switching rules. (For the relationship between AQL, LQ, CRP and PRP, see Figures 1 and 2 of
ISO/TR 8550-1:2007.)
In sequential sampling, items are selected randomly and inspected one after another, and a cumulative count
is kept of the number of nonconforming items or number of nonconformities. The decision to classify the lot as
acceptable or not acceptable can occur at almost any stage and, for sequential sampling by attributes,
depends on the number of items inspected and the cumulative number of nonconforming items or
nonconformities found up to that point.
ISO 2859-5 provides procedures, based on a sequential assessment of inspection results, that encourage a
supplier to supply lots of a good quality with a high probability of acceptance while maintaining an upper limit
for the risk of consumers accepting lots of poor quality.
4.7 ISO 2859-10: Sampling procedures for inspection by attributes — Part 10: Introduction
to the ISO 2859 series of standards for sampling for inspection by attributes
This is a companion document to this part of ISO/TR 8550 and can be read in conjunction with it, if desired,
but this is not essential. It is not a source of sampling schemes or plans.
Clause 4 of ISO 2859-10:2006 is essentially an introduction to the sampling schemes employed in ISO 2859-1,
ISO 2859-2 and ISO 2859-3, but it treats the subject in a general way. It contains explanations of terms, gives
practical advice on sampling inspection and discusses some underlying concepts. Clause 5 of
ISO 2859-10:2006 provides general and specific details of part 1 to part 5 in the ISO 2859 series, together
with an example of the use of each part.
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ISO/TR 8550-2:2007(E)
4.8 ISO 8422: Sequential sampling plans for inspection by attributes
ISO 8422 presents a sampling system that provides a wide range of sequential s
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