SIST ISO 3951-1:2006

INTERNATIONAL ISO
STANDARD 3951-1
First edition
2005-04-01
Sampling procedures for inspection by
variables —
Part 1:
Specification for single sampling plans
indexed by acceptance quality limit (AQL)
for lot-by-lot inspection for a single
quality characteristic and a single AQL
Règles d'échantillonnage pour les contrôles par mesures —
Partie 1: Spécifications pour les plans d'échantillonnage simples
indexés d'après le niveau de qualité acceptable (NQA) pour le contrôle
lot par lot pour une caractéristique de qualité unique et un NQA unique

Reference number
©
ISO 2005
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ii © ISO 2005 – All rights reserved

Contents Page
Foreword. v
Introduction . vii
1 Scope. 1
2 Normative references . 2
3 Terms and definitions. 2
4 Symbols . 5
5 Acceptance quality limit (AQL). 7
5.1 Principle . 7
5.2 Use. 7
5.3 Specifying AQLs . 7
5.4 Preferred AQLs. 7
5.5 Caution. 7
5.6 Limitation . 7
6 Switching rules for normal, tightened and reduced inspection. 7
7 Relation to ISO 2859-1 . 8
7.1 Similarities . 8
7.2 Differences. 8
8 Limiting quality protection. 9
8.1 Use of individual plans. 9
8.2 Consumer's risk quality tables . 9
8.3 Producer's risk tables. 9
8.4 Operating characteristic curves . 9
9 Planning . 10
10 Choice between variables and attributes .10
11 Choice between the “s” and “σσσσ” methods . 11
12 Choice of inspection level and AQL. 11
13 Choice of sampling plan. 11
13.1 Standard plans . 11
13.2 Special plans . 12
14 Preliminary operations . 12
15 Standard procedure for the “s” method. 12
15.1 Obtaining a plan, sampling and preliminary calculations . 12
15.2 Acceptability criteria for single specification limits. 13
15.3 Graphical method for a single specification limit. 14
15.4 Acceptability criterion for combined control of double specification limits . 15
16 Standard procedure for the “σσσσ” method. 21
16.1 Obtaining a plan, sampling and preliminary calculations . 21
16.2 Acceptability criteria for a single specification limit. 21
16.3 Acceptability criterion for combined control of double specification limits . 22
17 Procedure during continuing inspection.23
18 Normality and outliers . 24
18.1 Normality. 24
18.2 Outliers. 24
19 Records .24
19.1 Control charts.24
19.2 Lots that are not accepted.24
20 Operation of switching rules.24
21 Discontinuation and resumption of inspection.25
22 Switching between the “s” and “σσ” methods.25
σσ
22.1 Estimating the process standard deviation.25
22.2 State of statistical control .26
22.3 Switching from the “s” method to the “σσσσ” method.26
22.4 Switching from the “σσσσ” method to the “s” method.26
23 Chart A — Sample-size code letters of standard single sampling plans for specified
quality levels .26
24 Charts B to R (Figures 5 to 19) — Operating characteristic curves and tabulated values for
sample-size code letter B to R: “s” method .28
25 Charts s-D to s-R (Figures 20 to 32) — Acceptance curves for combined control of double
specification limits: “s” method.56
Annex A (normative) Tables for determining the appropriate sample size.69
Annex B (normative) Form k single sampling plans for the “s” method .71
Annex C (normative) Form k single sampling plans for the “σ” method .75
Annex D (normative) Values of f for maximum sample standard deviation (MSSD) .79
s
Annex E (normative) Values of f for maximum process standard deviation (MPSD) .83
σ
Annex F (normative) Estimating the process fraction nonconforming for sample size 3: “s”
method.84
Annex G (normative) Type p* single sampling plans.87
Annex H (normative) Values of c for upper control limit on the sample standard deviation .88
u
Annex I (normative) Supplementary acceptability constants for qualifying towards reduced
inspection.89
Annex J (normative) Procedures for obtaining s and σ .90
Annex K (informative) Consumer's risk qualities.92
Annex L (informative) Producer's risks .96
Annex M (informative) Operating characteristics for the “σ” method.100
Annex N (informative) Estimating the process fraction nonconforming for sample sizes 3
and 4 — “s” method.101
Bibliography.103

iv © ISO 2005 – All rights reserved

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
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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 3951-1 was prepared by Technical Committee ISO/TC 69, Applications of statistical methods,
Subcommittee SC 5, Acceptance sampling.
This first edition of ISO 3951-1 cancels and replaces ISO 3951:1989, of which it constitutes a technical
revision. The most significant differences between ISO 3951-1:2003 and ISO 3951:1989 are as follows.
 The acronym AQL now stands for Acceptance Quality Limit rather than Acceptable Quality Le
...