Measurement Uncertainty

1. Measurement Uncertainty Alex Williams In:Wenclawiak, Koch, Hadjicostas (eds.) Quality Assurance in Analytical Chemistry – Training and Teaching

2. Uncertainty • Is a fundamental property of a result • It is not an optional extra • It is not just as an additional burden • All results have an uncertainty on their value • Needs to be evaluated irrespective of requirements of 17025 In:Wenclawiak, Koch, Hadjicostas (eds.) Quality Assurance in Analytical Chemistry – Training and Teaching

3. Overview • What is meant by Uncertainty? • What information is it intended to give ? • Why is it important? • How is uncertainty evaluated? In:Wenclawiak, Koch, Hadjicostas (eds.) Quality Assurance in Analytical Chemistry – Training and Teaching

4. What is Uncertainty? • There will always be an uncertainty about the value of a result • Even when correction factors have been applied • Because there will be an uncertainty on these factors • There will also be an uncertainty arising from random effects. In:Wenclawiak, Koch, Hadjicostas (eds.) Quality Assurance in Analytical Chemistry – Training and Teaching

5. Lab A Lab B Lab C Lab D Lab E In:Wenclawiak, Koch, Hadjicostas (eds.) Quality Assurance in Analytical Chemistry – Training and Teaching

6. Units Lab A Lab B Lab C Lab D Lab E In:Wenclawiak, Koch, Hadjicostas (eds.) Quality Assurance in Analytical Chemistry – Training and Teaching

7. Why Uncertainty is Important • to assess the reliability of the result • to know the confidence that can be placed in any decisions based on its use • in order to compare measurement results In:Wenclawiak, Koch, Hadjicostas (eds.) Quality Assurance in Analytical Chemistry – Training and Teaching

8. Uncertainty should be Quantified in a Way that is • Universal:applicable to all kinds of measurements • Internally consistent:independent of how components are grouped • Transferable:use uncertainty on a result in derivation of uncertainty on dependant results • Procedures set out in: Guide to the Expression of Uncertainty in Measurement(GUM)Published by ISO ISBN 92-67-10188-9 1993 In:Wenclawiak, Koch, Hadjicostas (eds.) Quality Assurance in Analytical Chemistry – Training and Teaching

9. Uncertainty of Measurement - Definition • parameter, associated with the result of a measurement, that characterizes the dispersion of the values that could reasonably be attributed to the measurand In:Wenclawiak, Koch, Hadjicostas (eds.) Quality Assurance in Analytical Chemistry – Training and Teaching

10. Uncertainty of Measurement In:Wenclawiak, Koch, Hadjicostas (eds.) Quality Assurance in Analytical Chemistry – Training and Teaching

11. Standard Uncertainty • Uncertainty of the result expressed as a Standard Deviation In:Wenclawiak, Koch, Hadjicostas (eds.) Quality Assurance in Analytical Chemistry – Training and Teaching

12. Expanded Uncertainty • ..interval about the result of a measurement that may be expected to encompass a large fraction of the distribution of values that could reasonably be attributed to the measurand. In:Wenclawiak, Koch, Hadjicostas (eds.) Quality Assurance in Analytical Chemistry – Training and Teaching

13. Principles of Measurement • Measurement results are obtained from a comparison of the unknown with a standard • In the simplest case, the value VU of the unknown is VU=f(V1, IU, I1),Where • I1 indication, when measuring the standard, • IU when measuring the unknown and • V1 the value of the standard In:Wenclawiak, Koch, Hadjicostas (eds.) Quality Assurance in Analytical Chemistry – Training and Teaching

14. Evaluating Uncertainty • Uncertainty on result is:- • Uncertainty from comparison with standard. • Plus the uncertainty on the value of the standard. • Many sources of uncertainty associated with the comparison • Uncertainty on standard usually small • Traceability is to the value of the standard In:Wenclawiak, Koch, Hadjicostas (eds.) Quality Assurance in Analytical Chemistry – Training and Teaching

15. Measurement of Massusing two pan balance • Effects to be taken into account: • buoyancy • equality of arm lengths • sensitivity to position on pan • air temperature gradients • electrostatic forces In:Wenclawiak, Koch, Hadjicostas (eds.) Quality Assurance in Analytical Chemistry – Training and Teaching

16. Evaluating UncertaintySources of uncertainty • Incomplete definition of the measurand. • Sampling - the sample measured may not be representative. • Incomplete implementation of the measurement method. • Personal bias in reading analogue instruments. In:Wenclawiak, Koch, Hadjicostas (eds.) Quality Assurance in Analytical Chemistry – Training and Teaching

17. Evaluating UncertaintySources of uncertainty • Inadequate knowledge of the effects of environmental conditions on the measurement procedure or imperfect measurement of environmental conditions • Instrument calibration uncertainty In:Wenclawiak, Koch, Hadjicostas (eds.) Quality Assurance in Analytical Chemistry – Training and Teaching

18. Evaluating UncertaintySources of uncertainty • Instrument resolution or discrimination threshold. • Values assigned to measurement standards and reference materials. • Values of constants and other parameters obtained from external sources and used in the data reduction algorithm. In:Wenclawiak, Koch, Hadjicostas (eds.) Quality Assurance in Analytical Chemistry – Training and Teaching

19. Evaluating UncertaintySources of uncertainty • Instrumentapproximations and assumptions incorporated in the measurement method and procedure. • Variations in repeated observations of the measurand under apparently identical conditions. In:Wenclawiak, Koch, Hadjicostas (eds.) Quality Assurance in Analytical Chemistry – Training and Teaching

20. Evaluating Uncertainty • Approach depends on information available • Cause and effect diagram helps track components. In:Wenclawiak, Koch, Hadjicostas (eds.) Quality Assurance in Analytical Chemistry – Training and Teaching

21. Preparation of a Calibration Standard In:Wenclawiak, Koch, Hadjicostas (eds.) Quality Assurance in Analytical Chemistry – Training and Teaching

22. Pesticide in Bread In:Wenclawiak, Koch, Hadjicostas (eds.) Quality Assurance in Analytical Chemistry – Training and Teaching

23. Evaluating Uncertainty • For many types of test, method validation provides most of required information • Identifies and controls parameters that affect result • Plus normal QA In:Wenclawiak, Koch, Hadjicostas (eds.) Quality Assurance in Analytical Chemistry – Training and Teaching

24. Conclusions • Uncertainty is an essential component of the result • Necessary to ensure comparability of results • In many cases method validation studies & QA data provide most of information required • More information onwww.measurementuncertainty.org In:Wenclawiak, Koch, Hadjicostas (eds.) Quality Assurance in Analytical Chemistry – Training and Teaching