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“Uncertainty-noise” Le Mans
9
Acoustique
&
Techniques n° 40
validation method study. Repeatability and reproducibility
can be obtained by organising experimental work inside
the laboratory. Information can also be obtained from
quality control data (control chart).
The first difficulty is to include during the repetition of
the experiment a majority of influence quantities that can
affect the result.
The second difficulty is to assess the bias (or trueness of
the method). The use of CRMs, comparison with definitive
or reference method can be a solution to evaluate the
component of uncertainty related to the trueness.
The “interlaboratory validation approach”
When the model describing the measuring process
is not available, for any reasons, the major sources of
variability can often be assessed by inter-laboratory
studies stated in ISO 5725 “Accuracy (trueness and
precision) of measurement methods and results” which
provide estimates of repeatability (repeatability standard
deviation s
r
), reproducibility (reproducibility standard
deviation s
R
) and sometimes trueness of the method
(measured as a bias with respect to a known reference
value). This approach is fully described in ISO / TS 21748
[7] “Guidance for the use of repeatability, reproducibility
and trueness estimates in measurement uncertainty
estimation”.
The “PT approach”
Proficiency Test (PT) are intended to check periodically
the overall performance of a laboratory. The laboratory’s
results from its participation in proficiency testing can
accordingly be used to check the evaluated uncertainty,
since that uncertainty should be compatible with the
spread of results obtained by that laboratory over a
number or proficiency test rounds.
The “PT approach” can also be used to evaluate the
uncertainty. If the same method was used by all the
participants to the PT scheme, then the standard deviation
extract from individual results could be considered as a
preliminary evaluation of the measurement uncertainty.
For an individual laboratory, the bias and its uncertainty
combined with the within laboratory reproducibility may be
used as an estimate of the measurement uncertainty.
Combination of different approaches
In fact, very often a combination of the different approaches
needs to be used to assess the uncertainty. For example
when a laboratory decides to use the modelling approach,
the repeatability of the measuring process can be
assessed by using a quality control chart which provides a
good estimate of the within laboratory reproducibility. The
use of inter-laboratory validation approach can require
the application (by the CRM supplier) of the modelling
approach to evaluate the uncertainty on the reference
value of the CRM used to estimate the trueness of the
method.
If the measurand includes sampling, then mixing methods
for evaluating uncertainty due to sampling and test will be
appropriate.
Conclusion
In a recent past, testing laboratories were arguing against
the difficulties for the evaluation of uncertainties. A lot of
them were reluctant to the application of the propagation
law of uncertainty and its apparent mathematical
complexity.
The different approaches presented in this paper show
several possibilities for laboratories and particularly those
based on inter laboratory comparisons.
But the difficulties remain the same, whatever the method
used to evaluate the uncertainty of the result. It is essential
to define clearly the measurand and to analyse carefully
the measuring process in order to identify the factors
which influence the result. These two tasks requires more
technical competencies in measurement techniques than
mathematical skills.
The new approaches for uncertainty evaluation will
certainly bring facilities in the presentation of results, their
comparability and their traceability to SI Units.
Bibliography
[1] BIPM, IEC, IFCC, ISO, IUPAC, IUPAP, OIML - Guide to the expression of
uncertainty in measurement, ISO 1995
[2] JCGM, Guide to the expression of uncertainty in measurement supplement 1-
numerical methods for the propagation of distributions, to be published
[3] Christophe PERRUCHET et Marc PRIEL, Estimer l’incertitude – Mesures &
Essais, AFNOR, 2000 Spanish version published by AENOR 2000
[4] EA – 4/16 – Guidelines on the expression of uncertainty in quantitative
testing, European co-operation of accreditation, 2004 French and English
version available on the LNE web site www.lne.fr
[5] ISO 5725, Accuracy (trueness and precision) of measurement methods and
results, ISO 1994
[6] ISO/IEC 43-1 Proficiency testing by interlaboratory comparisons – Part 1
Development and operation of proficiency testing schemes, ISO 1997
[7] ISO / TS 21748 – Guidance for the use of repeatability, reproducibility and
trueness estimates in measurement uncertainty estimation, ISO 2004
[8] Eurolab Technical Report N° 1/2002 June 2002 “Measurement Uncertainty in
Testing” EUROLAB 2002
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The approaches for measurement uncertainties evaluation