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5.5 Key Comparisons & Interlaboratory Studies

W.F. Guthrie, K.R. Eberhardt, J.J. Filliben, S.D. Leigh,
M. Levenson, W. Liggett, J. Wang
Statistical Engineering Division, ITL

Background: Interlaboratory studies have long been used to ensure measurement capability for commerce since accurate measurements are necessary for assessing product specifications. For this reason design and analysis of interlaboratory studies have been an important part of the Statistical Engineering Division's (SED's) work for many years. Recently, however, a new type of interlaboratory study, known as a key comparison, has taken a critical new place in the NIST mission. In the last year, key comparisons, international interlaboratory studies for comparing measurement results between leading National Metrology Institutes (NMI's), have provided many new opportunities for SED to collaborate with scientists across NIST. The impetus for these new opportunities is a Mutual Recognition Arrangement (MRA) signed by the NMI's belonging to the International Committee for Weights and Measures (CIPM) ``to establish the degree of equivalence of national measurement standards maintained by NMIs, to provide for the mutual recognition of calibration and measurement certificates issued by NMIs, [and] thereby to provide governments and other parties with a secure technical foundation for wider agreements related to international trade, commerce and regulatory affairs.''

Goals: Key comparisons serve as the technical basis for the MRA and must therefore accurately reflect the true relationships between measurement systems maintained by NMI's belonging to the CIPM. The results of key comparisons must also be extensible to members of Regional Metrology Organizations (RMO's) to maximize recognition of measurement capabilities that exist in other metrology laboratories around the world. Impacts: The success of the MRA, based on successful key comparisons, will reduce measurement-based barriers to international trade by promoting convergence of measurement techniques. Assessment and discussion of measurement capabilities in an open forum may encourage NMI's to improve absolute measurement capabilities as well.

Customers: Customers of key comparisons include NMI's in the CIPM and the various RMO's, secondary government and commercial metrology laboratories, and end-users of secondary metrology labs' services.

SED Contributions: SED contributions to interlaboratory studies and key comparisons include data analyses which account for covariances in the measurements used to compute differences in results between labs and which ensure that uncertainties have a confidence level of 95%, as outlined in MRA policies. SED is also making contributions in the computation of reference values from interlaboratory data. Reference values are a sometimes controversial part of the MRA whose real role is still currently being worked out at both the policy and technical levels. The earliest key comparisons, those analyzed to date, have often used designs which allow for straight-forward computation of the necessary measurement differences but give less consideration to computation of uncertainties. Based on contacts from current work, however, SED has recently begun to contribute to comparison designs, ensuring that data collection will be as effective as possible. Opportunities to impact the design and analysis of comparisons have also been bolstered by participation in international meetings on interlaboratory studies organized by statisticians and other researchers from NMI's. As more key comparisons are finished there is also likely to be a need for SED to answer questions about the linkages between key comparisons and the measurement servies that they are designed to support.

Current Projects: The recent signing (October, 1999) of the MRA has already led to a large increase in the number of interlaboratory studies that SED is involved in. Current projects that SED members are contributing to are listed below. The fact that international recognition of measurement services that the NMI's offer is directly supported by regular participation in relevant key comparisons suggests that the MRA will be an increasingly important source of SED activity for some time.

Comparison Participants Customer Stage
Thermal Conductivity USA, UK, France, Canada, Japan BFRL Analysis
Temperature USA, Germany, Australia, France, China, Italy, Canada, Netherlands, Russia, UK, New Zealand, Korea, Japan, Slovakia CSTL Analysis
Humidity Standards USA, UK, Japan, Italy CSTL Design
Pressure - Absolute USA, Germany, Italy, UK, Australia, India, Korea CSTL Analysis
Pressure - Differential USA, Italy, UK, New Zealand, Australia CSTL Analysis
Capacitance Standards USA, Germany, Australia, France, UK, Netherlands, Russia, New Zealand, Canada EEEL Analysis
Basic Electrical Units Most countries in North, Central and South America, some Caribbean countries EEEL Analysis
Josephson Voltage Standard USA, Mexico, Canada and 12 secondary metrology laboratories EEEL Analysis
Magnetic Thin Film Properties NIST and 11 other secondary metrology labortories EEEL Analysis
Accelerometers USA, Canada, Mexico, Argentina, Brazil MEL Analysis
Lab. Standard Microphones USA, Canada, Mexico, Argentina, Brazil MEL Design
Lab. Standard Microphones Australia, Japan, Korea, Russia, Poland, UK, Germany, Denmark, Mexico, USA, Canada, New Zealand MEL Design
Vibration Acceleration Australia, Japan, Korea, Russia, Poland, Germany, Denmark, Mexico, USA, Canada, New Zealand MEL Design
Ultrasonic Power Australia, India, Russia, Germany, UK, USA, Canada MEL Design
Rockwell Hardness USA, Japan MSEL Design

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Date created: 7/20/2001
Last updated: 7/20/2001
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