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BS ISO 3951-2:2013:2018 Edition

BS ISO 3951-2:2013:2018 Edition

$215.11

Sampling procedures for inspection by variables – General specification for single sampling plans indexed by acceptance quality limit (AQL) for lot-by-lot inspection of independent quality characteristics

Published By Publication Date Number of Pages
BSI 2018 100
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This part of ISO 3951 is primarily designed for use under the following conditions:

  1. where the inspection procedure is to be applied to a continuing series of lots of discrete products all supplied by one producer using one production process. If there are different producers or production processes, this part of ISO 3951 shall be applied to each one separately;

  2. where the quality characteristics of the items of product are measurable on a continuous scale;

  3. where the measurement error is negligible (i.e. with a standard deviation no more than 10 % of the corresponding process standard deviation). However, procedures are also provided in Clause 9 and Annex P for accommodating measurement error when it has a non-negligible standard deviation;

  4. where production is stable (under statistical control) and the quality characteristics are distributed, at least to a close approximation, according to normal distributions;

  5. where, in the case of multiple quality characteristics, the characteristics are independent, or almost independent, of one another;

  6. where a contract or standard defines a lower specification limit, L, an upper specification limit, U, or both on each of the quality characteristics. If there is only one quality characteristic, an item is qualified as conforming if its measured quality characteristic x satisfies the appropriate one of the following inequalities:

    1. x ≥ L (i.e. the lower specification limit is not violated);

    2. x ≤ U (i.e. the upper specification limit is not violated);

    3. x ≥ L and x ≤ U (i.e. neither the lower nor the upper specification limit is violated).

  7. If there are two or more, say m, quality characteristics, then, designating the lower and upper limits for the ith quality characteristic by Li and Ui respectively, an item of product is qualified as nonconforming if one or more of its m measured quality characteristics, xi, fails to satisfy the appropriate one of the following inequalities:

    1. xi ≥ Li;

    2. xi ≤ Ui;

    3. xi ≥ Li and xi ≤ Ui.

Inequalities 1), 2), 4), and 5) are called cases with a single specification limit while 3) and 6) are called cases with double specification limits. For double specification limits, a further distinction is made between combined control, separate control, and complex control. If there is only one quality characteristic, then

  • combined control is where a single AQL applies to nonconformity beyond both limits,

  • separate control is where separate AQLs apply to nonconformity beyond each of the limits, and

  • complex control is where one AQL applies to nonconformity beyond the limit that is of greater seriousness and a larger AQL applies to the total nonconformity beyond both limits.

If there are two or more quality characteristics, this generalizes as follows:

  • combined control is where nonconformity beyond both limits on a variable belongs to the same class, to which a single AQL applies;

  • separate control is where nonconformity beyond the two limits on a variable belongs to separate classes, to each of which a single AQL applies;

  • complex control is where nonconformity beyond the limit that is of greater seriousness belongs to one class to which a single AQL applies, and the total nonconformity beyond both limits belongs to another class to which a larger AQL applies.

Note that, in the case of two or more quality characteristics, nonconformity on more than one quality characteristic may belong to the same class.

PDF Catalog

PDF Pages PDF Title
2 undefined
8 Foreword
10 Introduction
13 1 Scope
14 2 Normative references
3 Terms and definitions
18 4 Symbols
4.1 Univariate symbols
20 4.2 Multivariate symbols
5 Acceptance quality limit (AQL)
5.1 Concept
5.2 Use
5.3 Specifying AQLs
21 5.4 Preferred AQLs
5.5 Caution
5.6 Limitation
6 Switching rules for normal, tightened, and reduced inspection
22 7 Relation to ISO 2859-1 and ISO 3951-1
7.1 Relation to ISO 2859-1
7.1.1 Similarities to ISO 2859-1
7.1.2 Differences from ISO 2859-1
23 7.2 Relation to ISO 3951-1
7.2.1 Similarities to ISO 3951-1
7.2.2 Differences from ISO 3951-1
24 8 Consumer protection
8.1 Use of individual plans
8.2 Consumer’s risk quality (CRQ) tables
8.3 Producer’s risk tables
8.4 Operating characteristic (OC) curves
9 Accommodating measurement variability
25 10 Planning
11 Choice between variables and attributes
26 12 Choice between the s-method and σ-method
13 Choice of inspection level and AQL
14 Choice of sampling scheme
14.1 Standard plans
27 14.2 Special plans
15 Preliminary operations
28 16 Standard procedures for the univariate s-method
16.1 Obtaining a plan, sampling, and preliminary calculations
16.2 Form k acceptability criterion for the s-method
30 16.3 Form p* acceptability criterion for the s-method
16.3.1 Introduction
16.3.2 Combined control for the s-method
36 16.3.3 Separate control for the s-method
16.3.4 Complex control for the s-method
17 Standard multivariate s-method procedures for independent quality characteristics
17.1 General methodology
37 17.2 Example
39 18 Standard univariate σ-method procedures
18.1 Obtaining a plan, sampling, and preliminary calculations
18.2 Acceptability criterion for a single specification limit or for double specification limits with separate control
40 18.3 Acceptability criterion for double specification limits with combined or complex control
42 19 Standard multivariate σ-method procedures for independent quality characteristics
19.1 General methodology
43 19.2 Example
44 20 Standard multivariate combined s-method and σ-method procedures for independent quality characteristics
20.1 General methodology
45 20.2 Example
47 21 Procedure during continuing inspection
22 Normality and outliers
22.1 Normality
22.2 Outliers
23 Records
23.1 Control charts
23.2 Lots that are not accepted
48 24 Operation of switching rules
25 Discontinuation and resumption of inspection
49 26 Switching between the s-method and σ-method
26.1 Estimating the process standard deviation
26.2 State of statistical control
26.3 Switching from the s-method to the σ-method
26.4 Switching from the σ-method to the s-method
50 Annex A (normative) Table for determining the sample size code letter
51 Annex B (normative) Form k single sampling plans: s-method
54 Annex C (normative) Form k single sampling plans: σ-method
57 Annex D (normative) Form p* single sampling plans: s-method
60 Annex E (normative) Form p* single sampling plans: σ-method
63 Annex F (normative) Values of fs for maximum sample standard deviation (MSSD)
66 Annex G (normative) Values of fσ for maximum process standard deviation (MPSD)
69 Annex H (normative) Estimating the process fraction nonconforming for sample size 3: s-method
72 Annex I (normative) Values of cU for upper control limit on the sample standard deviation
73 Annex J (normative) Supplementary acceptability constants for qualifying towards reduced inspection
74 Annex K (normative) Procedures for obtaining s and σ
76 Annex L (normative) Estimating the process fraction nonconforming
82 Annex M (informative) Consumer’s risk qualities
86 Annex N (informative) Producer’s risks
90 Annex O (informative) Operating characteristics for the σ-method
91 Annex P (informative) Accommodating measurement variability
97 Bibliography

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BS ISO 3951-2:2013
$215.11