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Measure uncertainty
1. Removing the 'Uncertainty' from
Measurement Uncertainty
Thursday, 28 August 2008
Dr Robert Symons
National Measurement Institute
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. Definitions – ISO/IEC Guide 99:2007
International 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
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4. Definitions – ISO/IEC Guide 99:2007
International 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
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6. Cl
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Cl O N
What is the NMI doing? Cl N
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Ketotriclabendazole
Measurement 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
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.
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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
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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.
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11. Consultative Committee for the Amount of
Substance – 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
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EU
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11
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Laboratory
12. Consultative Committee for the Amount of
Substance – 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
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EU
PR
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Laboratory
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
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14. Why do we need matrix Certified
Reference Materials?
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18. II.19. Acceptance of a lot or sublot and
interpretation 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.
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19. Interpretation of the measurement of uncertainty when
considering 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
uncertainty
Action: reject accept accept accept
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21. The mean value of the results of three aliquots for the ‘A’ Sample
for Threshold Substances minus the value of measurement
uncertainty determined by the Laboratory must exceed the relevant
Threshold……………………Adverse Analytical Finding or Atypical
Finding decisions shall be based on the mean of the measured
concentrations, taking into account the measurement uncertainty
with the coverage factor, k, and a level of confidence of 95%.
Reports and documentation shall give the mean concentration with
the associated uncertainty.
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22. Interpretation of an adverse analytical finding
taking into account measurement of uncertainty
by the World Anti-Doping Agency
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
uncertainty
Action: reject accept accept accept 22
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
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24. Analytical
Report
MEASUREMENT
UNCERTAINTY MODEL
QC -
Reproducibility
within laboratory
Rw
3.2 ± 0.1
Method and Laboratory Bias Reproducibility
•Certified reference material between
Sample arrives at •Interlaboratory comparison laboratories SR
the laboratory •Method validation
LABORATORY CUSTOMER
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25. What is the NMI doing?
Measurement Uncertainty
Training Team
– an overview of ISO's 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
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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
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27. National Measurement Institute
1 Suakin Street
Pymble NSW 2073
Australia
Phone: + 61 2 9449 0111
Email: info@measurement.gov.au
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