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Analysis of key comparisons

Working Group 8.42


Key comparisons are interlaboratory comparisons carried out regularly between National Metrology Institutes (NMIs) within the framework of the CIPM Mutual Recognition Arrangement (Opens external link in new windowMRA). Key comparisons enable the mutual recognition of calibrations, measurements, and test certificates of the NMIs and mark a major step in supporting international trade, commerce and regulatory affairs. The final report and the supporting technical data of each key comparison are stored and made publicly available at the key comparison data base  Opens external link in new windowKCDB of the Bureau International des Poids et Mesures (Opens external link in new windowBIPM). Fig. 1 shows a typical example of key comparison data.

Fig.1 Example data of a key comparison along with the key comparison reference value (KCRV). The blue results indicate control measurements made by the so-called pilot laboratory.


The analysis of key comparisons can be seen as a Opens external link in new windowMeta-Analysis in which the results reported by the participating laboratories are assessed. Fixed effects or random effects models are popular models applied in meta-analysis. One focus of research of PTB’s Working Group 8.42 is the development of Bayesian methods for the estimation of fixed and random effects models. This includes the assignment of vaguely informative or appropriate non-informative priors and the exploration of the properties of the resulting inference. Other topics of research are situations in which the common measurand shows a drift, or where the explanatory power of key comparisons is assessed in dependence on the stability of the common measurand and the uncertainties reported by the participating laboratories.



Publication single view


Title: Determination of reference values for optical properties of liquid phantoms based on Intralipid and India ink
Author(s): L. Spinelli, M. Botwicz, N. Zolek, M. Kacprzak, D. Milej, P. Sawosz, A. Liebert, U. Weigel, T. Durduran, F. Foschum, A. Kienle, F. Baribeau, S. Leclair, J.-P. Bouchard, I. Noiseux, P. Gallant, O. Mermut, A. Farina, A. Pifferi, A. Torricelli, R. Cubeddu, H.-C. Ho, M. Mazurenka, H. Wabnitz, K. Klauenberg, O. Bodnar, C. Elster, M. Bénazech-Lavoué, Y. Bérubé-Lauzière, F. Lesage, D. Khoptyar, A. A. Subash, S. Andersson-Engels, P. Di Ninni, F. Martelli and G. Zaccanti
Journal: Biomedical optics express
Year: 2014
Volume: 5
Issue: 7
Pages: 2037--53
Optical Society of America
DOI: 10.1364/BOE.5.002037
ISSN: 2156-7085
Web URL: http://www.osapublishing.org/viewmedia.cfm?uri=boe-5-7-2037&seq=0&html=true
Keywords: Medical optics instrumentation,Photon migration,Turbid media
Tags: 8.42,KC
Abstract: A multi-center study has been set up to accurately characterize the optical properties of diffusive liquid phantoms based on Intralipid and India ink at near-infrared (NIR) wavelengths. Nine research laboratories from six countries adopting different measurement techniques, instrumental set-ups, and data analysis methods determined at their best the optical properties and relative uncertainties of diffusive dilutions prepared with common samples of the two compounds. By exploiting a suitable statistical model, comprehensive reference values at three NIR wavelengths for the intrinsic absorption coefficient of India ink and the intrinsic reduced scattering coefficient of Intralipid-20% were determined with an uncertainty of about 2% or better, depending on the wavelength considered, and 1%, respectively. Even if in this study we focused on particular batches of India ink and Intralipid, the reference values determined here represent a solid and useful starting point for preparing diffusive liquid phantoms with accurately defined optical properties. Furthermore, due to the ready availability, low cost, long-term stability and batch-to-batch reproducibility of these compounds, they provide a unique fundamental tool for the calibration and performance assessment of diffuse optical spectroscopy instrumentation intended to be used in laboratory or clinical environment. Finally, the collaborative work presented here demonstrates that the accuracy level attained in this work for optical properties of diffusive phantoms is reliable.

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