Measurement Uncertainty A Measure of Confidence - National ...

Measurement Uncertainty
A Measure of Confidence
Chua Sze Wey
Head (Electrical Metrology) &
Principal Metrologist
National Metrology Centre

Agenda
• A touch on measurement uncertainty
• Evolution of the GUM
• Update on the GUM
& where you can get a copy for FREE !

The only certainty in life is uncertainty
http://m.mfrtech.com/articles/25030.html

International vocabulary of metrology — Basic and general concepts
and associated terms (VIM) -2008
measurement uncertainty (uncertainty of measurement, uncertainty)
non-negative parameter characterizing the dispersion of the quantity
values being attributed to a measurand, based on the information
used
• NOTE 1 Measurement uncertainty includes components arising from systematic effects, such as
components associated with corrections and the assigned quantity values of measurement
standards, as well as the definitional uncertainty. Sometimes estimated systematic effects
are not corrected for but, instead, associated measurement uncertainty components are
incorporated.
• NOTE 2 The parameter may be, for example, a standard deviation called standard
measurement uncertainty (or a specified multiple of it), or the half-width of an interval, having a
stated coverage probability.
• NOTE 3 Measurement uncertainty comprises, in general, many components. Some of these may
be evaluated by Type A evaluation of measurement uncertainty from the statistical distribution
of the quantity values from series of measurements and can be characterized by standard
deviations. The other components, which may be evaluated by Type B evaluation of
measurement uncertainty, can also be characterized by standard deviations, evaluated from
probability density functions based on experience or other information.
• NOTE 4 In general, for a given set of information, it is understood that the measurement
uncertainty is associated with a stated quantity value attributed to the measurand. A modification
of this value results in a modification of the associated uncertainty.

Measurement and uncertainty
• Real measurements are never made under perfect
conditions, flaws in the measurement may be visible or
invisible.
• “uncertainty” means doubt, and thus in its broadest
sense “uncertainty of measurement” means doubt about
the validity of the result of a measurement.
• Every measurement is subject to some uncertainty.
• A measurement result is only complete if it is
accompanied by a statement of the uncertainty in the
measurement.

Measurement and uncertainty
• Uncertainty of measurement is thus an expression of the
fact that, for a given measurand and a given result of
measurement of it, there is not one value but an infinite
number of values dispersed about the result
• The values are consistent with all of the observations
and data and one's knowledge of the physical world, and
that with varying degrees of credibility can be attributed
to the measurand.

Where do uncertainties come from?
– Measuring instrument
Errors, bias, ageing, wear, drift, readability, noise and many other problems.
– Item being measured
which may not be stable
– Measurement process
the measurement itself may be difficult to make.
– ‘Imported’ uncertainties
calibration of instrument
– Operator skill
some measurements depend on the skill and judgment of the operator.
– Sampling issues
the measurements you make must be properly representative of the process
you are trying to assess
– Environment
– Etc
temperature, air pressure, humidity and many other conditions

How to estimate uncertainty
• Uncertainties can be estimated using statistical analysis
of a set of measurements, and using other kinds of
information about the measurement process.
• There are established rules for how to calculate an
overall estimate of uncertainty from these individual
pieces of information.
• When the uncertainty in a measurement is evaluated
and stated, the fitness for purpose of the measurement
can be properly judged.

Measurement Uncertainty Statement
In1930’s, C.H. Meyers et al. at NBS and his colleagues
conducted an elaborate experiment to determine the
specific heat of ammonia.
After several years of hard work, they completed the
experiment and wrote a paper reporting their results.
Toward the end of their paper, Meyers declared:

Measurement Uncertainty Statement
“We think our reported value is good to 1 part in 10,000: we
are willing to bet our own money at even odds that is
correct to 2 parts in 10,000.
Furthermore, if by any chance our value is shown to be in
error by more that 1 part in 1000, we are prepared to eat
the apparatus and drink the ammonia.”
C.H. Meyers et al. (NBS 1930s)
"Round Table Discussion of Statement of Data and Errors,"
Nuclear Instruments Methods 112, 391 (1973).

The need of general rules for evaluating and
expressing uncertainty in measurement
• Reporting the result of a measurement requires a quantitative indication of
the quality of the result be given so that those who use it can assess its
reliability.
• Without such an indication, measurement results cannot be compared,
either among themselves or with reference values given in a specification or
standard.
• It is therefore necessary that there be a readily implemented, easily
understood, and generally accepted procedure for characterizing the quality
of a result of a measurement, that is, for evaluating and expressing its
uncertainty.
• A worldwide consensus on the evaluation and expression of uncertainty in
measurement would permit vast spectrum of measurement results in
science, engineering, commerce, industry, and regulation to be readily
understood and properly interpreted.
• Imperative that the method for evaluating and expressing uncertainty be
uniform throughout the world so that measurements performed in different
countries can be easily compared in this era of the global marketplace

Development of GUM
• Initiated by the International Committee on Weights and Measures
(CIPM) in 1977.
• Recommendation INC–1 (1980), ‘Expression of experimental
uncertainties’ by Working Group on the Statement of Uncertainties
convened by the International Bureau of Weights and Measures
(BIPM).
• CIPM approved the Recommendation in 1981, and reaffirmed it in
1986.
• International Organization for Standardization (ISO) formed
Technical Advisory Group on Metrology (TAG4) with 6 other
international organizations to develop a detailed guide based on the
Recommendation.
– BIPM,
– International Electrotechnical Commission (IEC),
– International Federation of Clinical Chemistry and Laboratory Medicine (IFCC),
– International Union of Pure and Applied Chemistry (IUPAC),
– International Union of Pure and Applied Physics (IUPAP) and
– International Organization of Legal Metrology (OIML).

Development of GUM
• Guide to the Expression of Uncertainty in Measurement’ (GUM) was
published in 1993 and reprinted with minor corrections in 1995.
• In 1997 a Joint Committee for Guides in Metrology (JCGM), chaired
by the Director of the BIPM, was created by the seven international
organizations that had originally prepared the GUM.
• The JCGM Working Group ‘Expression of uncertainty in
measurement’, has the task of promoting the use of the GUM and
preparing supplements for its broad application and assumed
responsibility for GUM from ISO TAG4.
• In 1998 a further organization joined these seven international
organizations, namely, the International Laboratory Accreditation
Cooperation (ILAC).

ISO/IEC Guide 98-3:2008 Uncertainty of measurement
Part 3: Guide to the expression of uncertainty in
measurement

JCGM 100 : 2008
Evaluation of measurement data- Guide to the
expression of uncertainty in measurement

OIML G 1-100
Evaluation of measurement data- Guide to the
expression of uncertainty in measurement

Steps to evaluate the overall uncertainty
1. Decide what you need to find out from your measurements.
2. Identify what actual measurements and calculations are needed to produce
the final result.
3. Carry out the measurements needed.
4. Estimate the contributing uncertainty of each input quantity that feeds into
the final result.
5. Evaluate each contribution by
– Type A evaluations - uncertainty estimates using statistics
– Type B evaluations - uncertainty estimates from any other information.
6. Express all uncertainties in similar terms
7. Decide whether the input quantities are independent of each other. If not,
then some extra calculations are needed.
8. Calculate the result of your measurement, including any known corrections
for things such as calibration.
9. Find the combined standard uncertainty from all the individual aspects.
10.Expand the uncertainty in terms of a coverage factor together with a
uncertainty interval, and state a level of confidence.
11.Write down the measurement result and the uncertainty

New Development
• Wide international dissemination and acceptance of the
GUM
• Its release in the mid-1990s has had a dramatic effect in
improving the evaluation and reporting of measurement
uncertainty
• Timely to revise the GUM
• The GUM revision will take the form of supplemental
guides.

Evaluation of measurement data –
Guide to the expression of uncertainty in measurement
• The fundamental reference document
JCGM 100:2008 Evaluation of measurement data – Guide to the
expression of uncertainty in measurement
• JCGM 104:2009 Evaluation of measurement data – An introduction
to the GUM and related documents
• JCGM 101:2008 Evaluation of measurement data – Supplement 1 to
the GUM – Propagation of distributions using a Monte Carlo method
• JCGM 102:2011 Evaluation of measurement data – Supplement 2 to
the GUM– Extension to any number of output quantities
• Evaluation of measurement data – The role of measurement
uncertainty in conformity assessment
• Evaluation of measurement data – Concepts and basic principles
• Evaluation of measurement data – Supplement 3 to GUM –
Modelling
• Evaluation of measurement data – Applications of the least-squares
method

‘Guide to the Expression of Uncertainty in
Measurement’
• Provides general rules for evaluating and expressing
uncertainty in measurement
• Applicable to a wide range of measurements and for use
within standardization, calibration, laboratory
accreditation and measurement services.
• Reference document for evaluating and expressing
uncertainty in a broad spectrum of measurements rather
than detailed, technology-specific instructions.
• May be necessary to develop particular
guidelines/examples based on the Guide that deal with
the problems peculiar to specific fields of measurement
or with the various uses of quantitative expressions of
uncertainty”

http://www.bipm.org/en/publications/guides/gum.html

End of Presentation
Any Question ?
Email: chua_sze_wey@nmc.a-star.edu.sg