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# Measure uncertainty

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### Measure uncertainty

1. 1. Removing the Uncertainty fromMeasurement UncertaintyThursday, 28 August 2008Dr Robert SymonsNational Measurement Institute
2. 2. Presentation Overview Definitions Chemical and Biological Metrology Branch Chemical Reference Materials Chemical Proficiency Testing Bioanalysis Chemical Reference Methods Case Study Aflatoxins in Food Products Drugs in Sport 2
3. 3. Definitions – ISO/IEC Guide 99:2007International Vocabulary of Metrology (VIM) Metrology – Science of measurement and its application Measurement – Process of experimentally obtaining one or more quantity values that can reasonably be attributed to a quantity Measurement result – Set of quantity values being attributed to a measurand together with any other available relevant information Measurand – quantity intended to be measured 3
4. 4. Definitions – ISO/IEC Guide 99:2007International Vocabulary of Metrology (VIM) Measurement Uncertainty (u) – Non-negative parameter characterising the dispersion of the quantity values being attributed to a measurand, based on the information used. Expanded Measurement Uncertainty (U) – Product of a combined standard measurement uncertainty and a factor larger than one. Coverage Factor – Number larger than one by which a combined standard measurement uncertainty is multiplied to obtain an expanded measurement uncertainty. – NOTE: A coverage factor is usually symbolised k and nominally k = 2 representing the 95% confidence interval. – U = u x2 4
5. 5. What is MeasurementUncertainty?Associated with the Result It is the number after the ± ! 3.2 ± 0.1 mg kg-1 5
6. 6. Cl H Cl O NWhat is the NMI doing? Cl N O H KetotriclabendazoleMeasurement Uncertainty Chemical Reference Materials Team – pure chemical substances certified in accordance with metrological principles using established, international best practice protocols ISO Guide 34:2000; – pure chemical certified reference materials (CRMs) are used to calibrate the measurement process and they all have common characteristics: – assigned values are accompanied by an uncertainty statement; – information is given on the methods used to assign values; – the intended use of the CRM is described; – the reported values of CRMs are, by definition, traceable to SI. – illicit drugs, sports drugs and agrichemicals. 6
7. 7. 7
8. 8. What is the NMI doing?Measurement Uncertainty Chemical Proficiency Testing Team – The principal aims of our program: – to provide testing laboratories with a tool to improve the accuracy and traceability of their chemical measurements; – to provide Australian laboratories and NATA with information on the current state of the practice in each area of analysis; – to evaluate laboratory capability and encourage improvements in laboratory methods and performance from state of the practice to fit for purpose‘; – to enable participating laboratories to assess their performance relative to domestic and international peer laboratories and hence to improve the comparability of results between laboratories and between countries; – to develop and promote a fit for purpose and affordable metrological approach to proficiency testing; – to provide practical advice and support to laboratories regarding traceability of chemical measurements and estimation of measurement uncertainties. – Food, environmental and illicit drugs. 8
9. 9. What is the NMI doing?Measurement Uncertainty Bioanalysis Team – Accuracy and precision in quantitative real-time PCR analysis – Criteria for method validation and development of uncertainty budgets are essential components of any analytical procedure to ensure fitness-for-purpose – Through the CCQM Working Group on Bioanalysis, NMI is participating in pilot studies aimed at identifying factors that contribute to the uncertainty associated with PCR measurements and developing approaches for improving measurement accuracy and precision – New research with digital PCR that may lead to primary methods development 9
10. 10. What is the NMI doing?Measurement Uncertainty Chemical Reference Methods Team – Primary method development; – High accuracy methods for both inorganic and organic analyses; – Traceable to The International System of Units (SI); – Methods based upon Isotope Dilution Mass Spectrometry; – Produce matrix certified reference materials (CRMs) e.g., pesticides in tomato puree and nitrofurans in prawns; – Provide reference values for NMI’s proficiency testing programs; – Determine the best estimate for the uncertainty of results; – Responsible for demonstrating Australia’s capabilities in CCQM international intercomparisons. 10
11. 11. Consultative Committee for the Amount ofSubstance – Metrology in Chemistry CCQM-K24 Cd in Rice 15.4 15.2 Amount content (nmol/g) 15.0 14.8 14.6 14.4 U = ± 1.7% 14.2 14.0 13.8 13.6 St Ko U Ja Sw U Au N G EU PR C K S et an h er pa re st i tz he m C Af ad ra a n er hi an ric rl a li a a na la y a nd 11 nd s Laboratory
12. 12. Consultative Committee for the Amount ofSubstance – Metrology in Chemistry CCQM-K24 Cd in Rice 20.0 19.0 Amount content (nmol/g) 18.0 17.0 16.0 15.0 14.0 U = ± 15% 13.0 12.0 11.0 10.0 St Ko U Ja Sw U Au N G EU PR C K S et an h er pa re st i tz he m C Af ad ra a n er hi an ric rl a li a a na la y a nd 12 nd s Laboratory
13. 13. 2000 European Study: Lead in wine n = 129 laboratories 40.6 Concentration in microgram/Litre 37.9 28 (22%) above 35.2 50% 50% 32.5 29.8 27.1 24.4 21.7 19.0 16.3 11 (9%) below - 50% 13.6 0 10 20 30 40 50 60 70 80 90 13
14. 14. Why do we need matrix CertifiedReference Materials? 14
15. 15. Aflatoxins in Milk Powder (without reference material) 1.4Mass fraction of aflatoxin 1.2 1 (ug/kg) 0.8 0.6 0.4 0.2 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 Lab No. 15
16. 16. Aflatoxin levels in Milk Powder (with reference material) 1.4 1.2Mass fraction of aflatoxin 1 (ug/kg) 0.8 0.6 0.4 0.2 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 Lab No. 16
17. 17. 17http://ec.europa.eu/food/food/chemicalsafety/contaminants/comm_dec_2006_504guidance_en.pdf
18. 18. II.19. Acceptance of a lot or sublot andinterpretation of results For groundnuts, nuts and dried figs intended for direct human consumption: – acceptance if none of the laboratory samples exceeds the maximum limit, taking into account the measurement uncertainty and the correction for recovery; – rejection if one or more of the laboratory samples exceeds the maximum limit; – beyond reasonable doubt taking into account the measurement uncertainty and correction for recovery*,* The measurement of uncertainty should be subtracted from the analytical result after correction for recovery. This result is the analytical result which should be used when judging compliance of a consignment with EU legislation. 18
19. 19. Interpretation of the measurement of uncertainty whenconsidering compliance with a statutory limit, where the circle is the analytical result.maximum limit (i) ( ii ) ( iii ) ( iv ) Result less Result Result below Result plus uncertainty above limit limit but limit uncertainty above limit but limit within below limit within uncertainty uncertaintyAction: reject accept accept accept 19
20. 20. 20
21. 21. The mean value of the results of three aliquots for the ‘A’ Samplefor Threshold Substances minus the value of measurementuncertainty determined by the Laboratory must exceed the relevantThreshold……………………Adverse Analytical Finding or AtypicalFinding decisions shall be based on the mean of the measuredconcentrations, taking into account the measurement uncertaintywith the coverage factor, k, and a level of confidence of 95%.Reports and documentation shall give the mean concentration withthe associated uncertainty. 21
22. 22. Interpretation of an adverse analytical finding taking into account measurement of uncertainty by the World Anti-Doping Agencymaximum limit (i) ( ii ) ( iii ) ( iv ) Result less Result Result below Result plus uncertainty above limit limit but limit uncertainty above limit but limit within below limit within uncertainty uncertaintyAction: reject accept accept accept 22
23. 23. RESPONSIBILITIES OF THE LABORATORY • to supply results that are fit-for-purpose • to supply expanded measurement uncertainty values associated with the result when requested by the client • not to interpret or comment on results • not to comment on sampling OF THE REGULATOR • to make the decision whether the result is compliant or not • to decide if sampling is representative of the lot 23
24. 24. Analytical Report MEASUREMENT UNCERTAINTY MODEL QC - Reproducibility within laboratory Rw 3.2 ± 0.1 Method and Laboratory Bias Reproducibility •Certified reference material betweenSample arrives at •Interlaboratory comparison laboratories SR the laboratory •Method validation LABORATORY CUSTOMER 24
25. 25. What is the NMI doing?Measurement Uncertainty Training Team – an overview of ISOs Guide to the Expression of Uncertainty in Measurement – modeling a measurement – identifying uncertainty components – basic statistical concepts – uncertainty calculations – practical exercises – useful spreadsheet functions – application in the workplace – sensitivity coefficients – sample calculations 25
26. 26. Conclusions NMI can provide pure substance reference materials NMI can provide matrix certified reference materials NMI can provide proficiency testing programs NMI can provide training 26
27. 27. National Measurement Institute1 Suakin StreetPymble NSW 2073AustraliaPhone: + 61 2 9449 0111Email: info@measurement.gov.au 27