1. C. Ehrlich and R. Dybkær Uncertainty of Error: The Error Dilemma Charles D. Ehrlich NIST Weights and Measures Division Physical Measurements Laboratory René Dybkær REGION H Frederiksberg Hospital Frederiksberg, Denmark NCSLI August 24, 2011; Session 8E

2. C. Ehrlich and R. Dybkær Question: What is the length of this pen? a) 10 cm? b) 15 cm? c) 20 cm? d) none of the above? Uncertainty of Error: The Error Dilemma NCSLI August 24, 2011; Session 8E

3. C. Ehrlich and R. Dybkær A related question: What is the (quantity) value of the length of this pen? a) 10 cm? b) 15 cm? c) 20 cm? Uncertainty of Error: The Error Dilemma NCSLI August 24, 2011; Session 8E

4. C. Ehrlich and R. Dybkær ‘Quantity’ and ‘quantity value’ are conceptually different: The concept of quantity does not require that a comparison (usually with a measurement unit) has actually taken place. The concept of quantity value does require that a comparison has taken place (before the quantity value can be known). Uncertainty of Error: The Error Dilemma NCSLI August 24, 2011; Session 8E

5. C. Ehrlich and R. Dybkær VIM3 definition of ‘quantity’: property of a phenomenon, body or substance, where the property has a magnitude that can be expressed as a number and a reference (e.g., spatial separation between the two ends of my pen) ********************************************************************** VIM3 definition of ‘quantity value’: number and reference together expressing magnitude of a quantity (e.g., 15 cm) Uncertainty of Error: The Error Dilemma NCSLI August 24, 2011; Session 8E

6. C. Ehrlich and R. Dybkær Why does this matter? VIM3 definition of ‘measurement error’: measured quantity value minus a reference quantity value Since values can’t be measured, neither can measurement error if it is defined this way. Uncertainty of Error: The Error Dilemma NCSLI August 24, 2011; Session 8E

7. C. Ehrlich and R. Dybkær VIM3 definition of ‘measurement error’: measured quantity value minus a reference quantity value “unknowable” measurement error reference quantity value is a true quantity value (it is not possible to assign a value to the measurement error) “knowable” (measurement) error reference quantity value is a conventional quantity value (it is possible to assign a value and a measurement uncertainty to the measurement error) Uncertainty of Error: The Error Dilemma NCSLI August 24, 2011; Session 8E

8. C. Ehrlich and R. Dybkær Uncertainty of Error: The Error Dilemma NCSLI August 24, 2011; Session 8E

9. C. Ehrlich and R. Dybkær Another question (not a trick question!): Match: ‘measurement uncertainty’ and ‘measurement error’ with their correct corresponding characterization: a) Measure of how well a true value of a measurand is actually known. b) Measure of how well a true value of a measurand is believed to be known. Uncertainty of Error: The Error Dilemma NCSLI August 24, 2011; Session 8E

10. C. Ehrlich and R. Dybkær Measurement Uncertainty Measure of how well a true value of a measurand is believed to be known (not a measure of how well a true value is actually known) Uncertainty of Error: The Error Dilemma NCSLI August 24, 2011; Session 8E

11. C. Ehrlich and R. Dybkær VIM3 definition of ‘measurement error’: measured quantity value minus a reference quantity value “unknowable” measurement error reference quantity value is a true quantity value (it is not possible to assign a value to the measurement error) “knowable” (measurement) error reference quantity value is a conventional quantity value (it is possible to assign a value and a measurement uncertainty to the measurement error) Uncertainty of Error: The Error Dilemma NCSLI August 24, 2011; Session 8E

12. C. Ehrlich and R. Dybkær Uncertainty of Error: The Error Dilemma NCSLI August 24, 2011; Session 8E

13. C. Ehrlich and R. Dybkær Uncertainty of Error: The Error Dilemma NCSLI August 24, 2011; Session 8E

14. C. Ehrlich and R. Dybkær Should “knowable” errors, such as ‘error of indication’ and ‘manufacturing error,’ be considered as ‘measurement errors’ in the VIM3 sense (and not in the sense of only resulting from imperfect measurements)? VIM3 definition of ‘measurement error’: measured quantity value minus a reference quantity value Uncertainty of Error: The Error Dilemma NCSLI August 24, 2011; Session 8E

15. C. Ehrlich and R. Dybkær The Error Dilemma: Should ‘measurement error’ be defined as a difference of quantities, instead of as a difference of quantity values? NCSLI August 24, 2011; Session 8E Uncertainty of Error: The Error Dilemma

16. C. Ehrlich and R. Dybkær Main Advantage: All types of error would be treated on the same footing, as being quantities, which would avoid dual usage of the term “error” for different concepts Possible Problem: Many texts that describe, discuss and use measurement error would need to be changed, along with the corresponding educational efforts NCSLI August 24, 2011; Session 8E Uncertainty of Error: The Error Dilemma

17. C. Ehrlich and R. Dybkær Time to stand up and stretch for 60 seconds Discuss with one of your neighbors: Which way you would solve the error dilemma…? i.e., Would you continue to define measurement error as a difference of quantity values, or instead as a difference of quantities? Uncertainty of Error: The Error Dilemma NCSLI August 24, 2011; Session 8E

18. C. Ehrlich and R. Dybkær Which way you would solve the error dilemma…? - Difference of quantity values? - Difference of quantities? - Not sure? Uncertainty of Error: The Error Dilemma NCSLI August 24, 2011; Session 8E

19. C. Ehrlich and R. Dybkær THANK YOU Uncertainty of Error: The Error Dilemma NCSLI August 24, 2011; Session 8E

20. C. Ehrlich and R. Dybkær GUM SUPPLEMENT: Evaluation of measurement data – An introduction to the “Guide to the expression of uncertainty in measurement” and related documents JCGM 104:2009 Uncertainty of Error: The Error Dilemma NCSLI August 24, 2011; Session 8E

21. C. Ehrlich and R. Dybkær GUM SUPPLEMENT: Evaluation of measurement data – Supplement 1 to the “Guide to the expression of uncertainty in measurement” – Propagation of distributions using a Monte Carlo method JCGM 101:2008 Uncertainty of Error: The Error Dilemma NCSLI August 24, 2011; Session 8E

22. C. Ehrlich and R. Dybkær GUM SUPPLEMENT: Evaluation of measurement data – The role of measurement uncertainty in conformity assessment JCGM 106:Due out soon Uncertainty of Error: The Error Dilemma NCSLI August 24, 2011; Session 8E

23. C. Ehrlich and R. Dybkær GUM SUPPLEMENT: Evaluation of measurement data – Concepts and basic principles Under development Uncertainty of Error: The Error Dilemma NCSLI August 24, 2011; Session 8E

24. C. Ehrlich and R. Dybkær GUM SUPPLEMENT: Evaluation of measurement data – Supplement 2 to the “Guide to the expression of uncertainty in measurement” – Models with any number of output quantities Under development Uncertainty of Error: The Error Dilemma NCSLI August 24, 2011; Session 8E