ISO 18414:2006

INTERNATIONAL ISO
STANDARD 18414
First edition
2006-01-15


Acceptance sampling procedures by
attributes — Accept-zero sampling
system based on credit principle for
controlling outgoing quality
Procédures d'échantillonnage par attributs pour acceptation — Système
d'échantillonnage de tolérance zéro-défaut basé sur le principe de
crédit pour le contrôle de la qualité à la sortie





Reference number
ISO 18414:2006(E)
©
ISO 2006

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ISO 18414:2006(E)
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ISO 18414:2006(E)
Contents Page
Foreword. iv
1 Scope .1
2 Normative references .2
3 Terms and definitions .2
4 Symbols and abbreviated terms .2
5 Average outgoing quality limit .3
6 Credit principle.3
7 Treatment of non-accepted lots .4
8 Sampling plans .4
9 Preliminary operations.4
10 Standard procedure.5
11 Procedure during continuing inspection .6
12 Discontinuation of inspection .6
Annex A (informative) Examples of sample sizes required for credit-based accept-zero plans .8
Bibliography .9

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ISO 18414:2006(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
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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.
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 18414 was prepared by Technical Committee ISO/TC 69, Applications of statistical methods,
Subcommittee SC 5, Acceptance sampling.

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INTERNATIONAL STANDARD ISO 18414:2006(E)

Acceptance sampling procedures by attributes — Accept-zero
sampling system based on credit principle for controlling
outgoing quality
1 Scope
1.1 This International Standard specifies a system of single sampling schemes for lot-by-lot inspection by
attributes. All the sampling plans of the present system are of accept-zero form, i.e. no lot is accepted if the
sample from it contains one or more nonconforming items. The schemes depend on a suitably-defined
average outgoing quality limit (AOQL), the value of which is chosen by the user; no restrictions are placed on
the choice of the value of the AOQL or on the sizes of successive lots in the series. The methodology ensures
that the overall average quality reaching the customer or market-place will not exceed the AOQL in the long
run.
1.2 The schemes are intended to induce a supplier, through the economic and psychological pressure of lot
non-acceptance and consequent loss of accumulated credit, to attempt to maintain a nonconformity-free
process, while assuring, by means of the lowest practicable sample sizes, that the long-term percentage of
nonconforming items delivered to the customer or market-place does not exceed the AOQL. This objective is
achieved by a progressive reduction in the sample size in response to good quality history.
1.3 The schemes are designed to be applied to a series of lots from each supplier. The credit principle
provides:
a) automatic protection to the customer if a deterioration in quality is detected, by means of a total loss of
accumulated credit and reversion to a relatively large sample size whenever a nonconforming item is
found, and 100 % inspection of
1) the first lot if it is not accepted, or
2) any non-accepted lot that immediately follows a non-accepted lot;
b) an incentive to reduce sampling costs (by means of a progressive reduction in required sample size) if
consistently good quality is achieved.
1.4 This International Standard is designed for use under the following conditions:
a) where the inspection procedure is to be applied to a series of lots of discrete items that are intended to be
identical, and which are all supplied by one producer using one production process (If there are different
producers or production processes, this International Standard is intended to be applied to each one
separately.);
b) where one or more quality characteristics of these products are taken into consideration, which must all
be classifiable as either conforming or nonconforming;
c) where the inspection error involved in classifying the state of a product's quality characteristic(s) is
negligible;
d) where inspection is non-destructive.
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ISO 18414:2006(E)
This International Standard can be suitable for regulatory purposes, as control of the expected quality of items
reaching the market-place is achieved with the smallest possible sample sizes, and long-term control of the
realized, or actual quality level in the market-place is achieved with certainty, regardless of how long or short
individual suppliers' series may be. This International Standard can be used by suppliers/producers,
buyers/consumers and regulatory agencies to provide control of the expected quality of the totality of accepted
product of each type from each source.
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 (including any amendments) applies.
ISO 3534-2, Statistics — Vocabulary and symbols — Part 2: Statistical quality control
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 3534-2 and the following apply.
3.1.1
credit
K
〈acceptance sampling〉 total number of items accepted in a sequence of accepted lots since a lot in the
sequence was non-accepted, or since the start of inspection if all previous inspected lots have been accepted
3.1.2
average outgoing quality
AOQ
〈acceptance sampling〉 long-term average quality level of outgoing product for a given value of incoming
product quality
NOTE In this International Standard, the average outgoing quality (AOQ) is calculated over all accepted lots together
with the conforming items found after 100 % inspection of lots that are non-accepted when the credit, K, is zero.
3.1.3
average outgoing quality limit
AOQL
a
〈acceptance sampling〉 maximum AOQ over all possible values of incoming product quality level for a given
acceptance sampling scheme.
4 Symbols and abbreviated terms
The symbols and abbreviations used in this International Standard are as follows:
AOQ average outgoing quality, in percent nonconforming items
AOQL average outgoing quality limit, in percent nonconforming items
a specified value of the AOQL, expressed in proportion of nonconforming items
d number of nonconforming items in the sample
K credit (an integer)
K upper limit (if any) to the usable credit
max
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ISO 18414:2006(E)
n sample size
N lot size
5 Average outgoing quality limit
When the quality of lots submitted for inspection is very good, almost all the lots are accepted and enter the
market-place unchanged. The average outgoing quality (AOQ) of items to the market-place is therefore a low
percentage nonconforming.
When the quality of lots submitted for inspection is very poor, most of the lots are non-accepted and 100 %
inspected. In this case, most of the items are subjected to inspection and only the conforming items accepted,
so the AOQ of items to the market-place is again a low percentage nonconforming, albeit at a relatively high
average inspection cost per item reaching the market-place.
As quality moves between these extremes, the AOQ increases, reaches a limit, and thereafter decreases. The
upper limit to the average outgoing percentage nonconforming is called the average outgoing quality limit
(AOQL). No sampling scheme obtained by use of this standard has an expected (or long-term) AOQ in excess
of the specified AOQL.
For an isolated lot or a short series of lots, there is a chance of the actual AOQ exceeding the AOQL. This
chance depends on the length of the series, the lot sizes (which are not necessarily constant) within the series,
and the quality levels in these lots, and tends to zero as the number of lots in the series increases.
CAUTION — It follows that plans from this International Standard are unsuitable for guaranteeing an
upper limit to the realized (as against expected) process fraction nonconforming for isolated lots or
short series of lots.
As sample sizes by their nature are constrained to be integers, the sampling schemes resulting from the
application of this International Standard tend to be slightly on the conservative side, resulting in an AOQL
slightly smaller than that specified.
The average outgoing quality after a long sequence of lot submissions only approaches the AOQL under the
most adverse sequence of lot qualities. In practice, this is unlikely to occur, providing another reason for the
AOQ to tend to be generally somewhat better than the specified AOQL.
NOTE The sampling procedure results only in the lot being unchanged or in the removal of its nonconforming items.
Hence, if the submitted lot quality is consistently much better than the AOQL, then the AOQ will also be much better than
the AOQL.
6 Credit principle
The unifying concept behind the sampling schemes in this International St
...