Sampling procedures in legal metrology

Working group 8.42

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Sampling procedures are used in industry, medicine and for election forecasts, among others. In legal metrology sampling plans are applied to verify the quantity of product in prepackages (see OIML R 87), for conformity assessment when placing utility meters on the market (see EU-Directive 2014/32/EC and OIML G 20) and subsequent verification of meters at predefined periodic intervals.

Two key elements of metrological quality control are the initial verification of measuring instruments before being placed on the market, and their subsequent verification at periodic intervals. Sampling procedures allow for efficient inspection of random subsets of these measuring instruments.

Hypothesis-based acceptance sampling for the MID

In the EU, the initial conformity assessment of measuring instruments is regulated by the Measuring Instruments Directive (MID). In particular, the modules F and F1 allow product verification on a “statistical basis”. PTB working group 8.42, together with the Bayerisches Landesamt für Maß und Gewicht, has re-interpreted the conditions formulated in the MID for these acceptance sampling procedures and contrasted them with the interpretation advanced in WELMEC guide 8.10. [Klauenberg et al., 2021, Müller and Klauenberg, 2021]. The new interpretation has economic as well as conceptual advantages, and is less ambiguous for finite and small lot sizes. For this new, hypothesis-based interpretation of the MID conditions sampling plans were developed, see [Klauenberg et al., 2021, Müller and Klauenberg, 2021] and made freely available in an interactive web-application as R Shiny app.

Sampling procedures for surveilling measuring instruments

Regular inspections are a key element of metrological quality control. One aim is, that 95 % of the utility meters fulfil conformance standards at all times. To guarantee this with high confidence and without inspecting every meter, PTB working group 8.42 develops procedures for regular sampling inspections. 

1. PTB working group 8.42 has developed a new statistical method, which combines standard sampling plans with a reliability function.  (See figure and Klauenberg and Elster, 2017)
2. In addition to developing this simple and generally applicable method, the mathematical assumptions for more efficient sampling plans were researched and published  (Klauenberg et.al., 2018).
3. The complementary methods 1) and 2) can be combined. One option offers high efficiency via small sample sizes and requires evidence that additional quality criteria are fulfilled. The alternative offers high flexibility via few assumptions. Both methods are readily realized by look-up tables (Klauenberg and Elster, 2018).

The aim of ongoing research is to develop new statistical approaches for a more efficient, more flexible and at the same time reliable surveillance of the quality of utility meters. Specifically, the benefit of additional information (so-called prior knowledge) shall be investigated when repeating reliability inspections and when implementing these in shorter time intervals. The methods of Bayesian experimental and adaptive design which are developed and investigated, shall identify new possibilities for repeated statistical quality control and at the same time provide the basis to suggest alternative sampling procedures, among others for section 35 of the Measures and Verification Ordinance.

Statistical verifications for a qualification procedure

The sampling procedures developed above offer the possibility to apply efficient sampling plans, if the population fulfils additional requirements (see also procedure 2 in Klauenberg and Elster, 2018). To statistically verify these requirements within a so-called qualification procedure, PTB working group 8.42 has selected and adapted statistical hypothesis tests to detect violations of the Normal distribution and of a limiting small failure rate. In addition, we recommended sample sizes for these tests. An introduction to Normality testing was prepared, which exemplifies this application (see also Klauenberg and Elster, 2019).

Statistical procedure enables enforcement of a change in the Measures and Verification Ordinance

Subsequent verification of utility meters is legally regulated in section 35 of the Measures and Verification Ordinance in Germany. To enforce the respective new regulation, the verification authorities have published the new administrative regulation “GM-VA SPV”. This regulation is based on the developed statistical procedures described above and on consultations carried out by PTB.

Starting 2019, a new procedure has entered into force which – by means of regular sampling inspections – will guarantee with high confidence that 95 % of the in-service measuring instruments fulfil the legal requirements at any time. Inspections carried out before then did not allow any statements about the quality of utility meters between such assessments. The new statistical procedure thus offers better consumer protection for water, electricity, gas and heat meters.

Statistical verifications enable utility meters to qualify for efficient sampling procedures

To apply efficient sampling plans according to section 35 of the German Measures and Verification Ordinance (see also section 4.3 in the administrative regulation Verfahrensanweisung für Stichprobenverfahren zur Verlängerung der Eichfrist), the associations for electricity-, gas-, water-, and heat meter industry have formulated a qualification procedure (see qualification procedure). This procedure is based on the statistical verifications developed at PTB, as well as on consultations carried out by PTB for the expert team „Qualifikationsverfahren zur Stichprobenprüfung“ of the Forum Netztechnik/Netzbetrieb (FNN) in the Verband der Elektrotechnik Elektronik Informationstechnik e.V. (VDE). To support the implementation of the qualification procedure, PTB working group 8.42 has also developed a specific Excel application, which realizes multiple tests to detect violations of the Normal distribution (see Software).

• Software