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Measurement Uncertainty

Measurement Uncertainty. Alex Williams. 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. Overview.

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Measurement Uncertainty

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  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

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