Guide for generating sampling plans for statistical verification ...

WELMEC 8.10 Issue 1

WELMEC European cooperation in legal metrology

Measuring Instruments Directive (2004/22/EC): Guide for generating sampling plans for statistical verification according to Annex F and F1 of MID 2004/22/EC

May 2011

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WELMEC European cooperation in legal metrology

WELMEC is a co-operation between the legal metrology authorities of the Member States of the European Union and EFTA. This document is one of a number of Guides published by WELMEC to provide guidance to manufacturers of measuring instruments and to Notified Bodies responsible for conformity assessment of their products. The Guides are purely advisory and do not themselves impose any restrictions or additional technical requirements beyond those contained in relevant EC Directives. Alternative approaches may be acceptable, but the guidance provided in this document represents the considered view of WELMEC as to the best practice to be followed. Published by: WELMEC Secretariat MIRS Grudnovo nabrežje 17 SI – 1000 Ljubljana e-mail : [email protected] Tel: +386 1 244 27 18 Fax: +386 1 244 27 14

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Contents 1

General

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Terms

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MID-conditions for statistical verification

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Creation of sampling plans fulfilling MID

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Examples

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Standards concerning Acceptance Sampling

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General Annex F of MID describes conformity assessment procedures, where a notified body chosen by the manufacturer carries out the appropriate examinations and tests, or have them carried out, to check the conformity of the instruments with the type as described in the EC-type examination certificate and the appropriate requirements of this Directive. Annex F1 of MID describes conformity assessment procedures, where a notified body carries out the appropriate examinations and tests, or have them carried out, to check the conformity of the instruments with the appropriate requirements of this Directive. Verification can be done by examination and testing of every instrument in accordance with MID Annex F, section 4 or Annex F1, section 6 or by statistical verification in accordance with MID Annex F, point 5 or Annex F1, point 7. Typical applications for statistical verification are material measures (Annex MI-008) or electronic electricity meters (Annex MI-003). Statistical verification is based on sampling plans agreed between notified bodies and manufacturers taking into account the provisions given in MID, Annex F or F1. This Guide is intended to support notified bodies when creating suitable sampling plans for conformity assessment acc. to Annex F and F1., statistical methods can also be used for Annexes A11 or for Annexes D, D1, E, E1, H and H12. The guide provides a practicable tool for the application of already existing documents and standards (in particular ISO 2859) with regard to MID conditions. This Guide is intended to improve understanding and applicability of standards for non-professionals and does make no claim to be complete.

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This annex requires product checks by the notified body. If appropriate the notified body may decide to create its sampling plans according to statistical methods. 2 These annexes do not include specific requirements for final product inspection by the manufacturer. The decision whether to perform testing of every instrument, statistical control of the process or the product remains with the manufacturer. In the latter case the notified body has to evaluate the sampling plans.

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Terms Sampling plan combination of sample size(s) to be used and associated lot acceptability criteria NOTE 1 A single sampling plan is a combination of sample size and acceptance and rejection numbers. A double sampling plan is a combination of two sample sizes and acceptance and rejection numbers for the first sample and for the combined sample. NOTE 2 A sampling plan does not contain the rules on how to draw the sample. NOTE 3 For the purposes of this part of ISO 2859, a distinction should be made between the terms sampling plan (3.1.17), sampling scheme (3.1.18) and sampling system (3.1.19). [ISO 2859-1:1999 , 3.1.17] Sampling scheme combination of sampling plans with rules for changing from one plan to another [ISO 2859-1:1999 , 3.1.18] Sampling system collection of sampling plans, or of sampling schemes, each with its own rules for changing plans, together with sampling procedures including criteria by which appropriate plans or schemes may be chosen NOTE This part of ISO 2859 is a sampling system indexed by lot-size ranges, inspection levels and AQLs. A sampling system for LQ plans is given in ISO 2859-2. [ISO 2859-1:1999 , 3.1.19] Operating characteristic curve (OC curve) A curve showing, for a given sampling plan, the probability of accepting a lot, as a function of the quality level of the process from which the lots come; also as used for some types of plans, a curve showing the percentage of lots, or product items, that may be expected to be accepted as a function of the process quality level NOTE: This type of operating curve is known as Type B. There are also operating characteristic curves of Type A and C. Producer’s risk For a given sampling plan, the probability of non-acceptance of a lot when the lot or process quality level (e.g. the fraction nonconforming) has a value stated by the plan as acceptable (e.g. an acceptable quality level, AQL) Consumer’s risk For a given sampling plan, the probability of acceptance of a lot or process when the quality level (e.g. fraction nonconforming) has a value stated by the plan as unsatisfactory (e.g. a limiting quality level, LQL)

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Acceptance quality limit AQL 3 quality level that is the worst tolerable process average when a continuing series of lots is submitted for acceptance sampling NOTE 1 This concept only applies when a sampling scheme with rules for switching and for discontinuation, such as in ISO 2859-1 or ISO 3951, is used. NOTE 2 Although individual lots with quality as bad as the acceptance quality limit may be accepted with fairly high probability, the designation of an acceptance quality limit does not suggest that this is a desirable quality level. Sampling schemes found in International Standards such as this part of ISO 2859, with their rules for switching and for discontinuation of sampling inspection, are designed to encourage suppliers to have process averages consistently better than the AQL. Otherwise, there is a high risk that the inspection severity will be switched to tightened inspection under which the criteria for lot acceptance become more demanding. Once on tightened inspection, unless action is taken to improve the process, it is very likely that the rule requiring discontinuation of sampling inspection pending such improvement will be invoked. [ISO 2859-1:1999 , 3.1.26] Limiting quality LQ 4 when a lot is considered in isolation, a quality level which for the purposes of sampling inspection is limited to a low probability of acceptance [ISO 2859-1:1999 , 3.1.28] p

is the proportion of nonconforming items; 5

Pa

probability of acceptance (in percent)

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In MID analogous to „level of quality“ In MID analogous to „limit quality“ 5 In MID analogous to „non-conformity“ 4

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MID-conditions for statistical verification Requirements for statistical verification are given in Annex F, point 5 or Annex F1, point 7 respectively. More specific requirements are laid down in Annex F, 5.3 or Annex F1, 7.3. The statistical control will be based on attributes. The sampling system shall ensure: — a level of quality corresponding to a probability of acceptance of 95 %, with a non-conformity of less than 1 %; — a limit quality corresponding to a probability of acceptance of 5 %, with a non-conformity of less than 7 %.

Explanations: The sampling schemes agreed between notified body and manufacturer may only consist of sampling plans, that fulfil both conditions. Both conditions refer to two specified points of the Operating characteristic (OC) curves of the sampling plan:



Pa (1 %) = 95%; Pa (7 %) = 5%;

(AQL) (LQ)

The first condition Pa (AQL) = 95%, p