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8
“Uncertainty-noise” Le Mans
Acoustique
&
Techniques n° 40
This classification is based on distinction between
uncertainty evaluation carried out by the laboratory
itself (called intra-laboratory approach) and uncertainty
evaluation based on collaborative studies (called inter-
laboratory approach). These approaches have been
presented in several documents [3], [4].
Both two approaches are then subdivided into:
- Modelling of the measurement process and use of the
propagation law of uncertainty and single laboratory
validation approach,
- Use of performance method data (ISO 5725) [5] and
Proficiency testing data (ISO-Guide 43) [6] for inter-
laboratory approaches.
Common points between the different approaches
Whatever the methods intended to be used, it is always
important:
- To define clearly, with no ambiguity the measurand or the
characteristic to be measured, analysed or tested.
- To analyse carefully the measuring or testing process in
order to identify the major components of uncertainty and
to examine if they are taken on board in the application
of the law of uncertainty or if they are active during
the repetition of observations organised to evaluate
repeatability and reproducibility or if they are included in
collaborative studies. It is also important to admit that
in some situations, we are not in a position to identify
the components of the uncertainty: the symptom of this
can be seen, when the uncertainty evaluated by applying
the GUM procedure leads to a value of much less large
uncertainty than the variation observed among different
laboratory intercomparisons.
Where sampling activities are performed, it is also
important to define clearly the measurand. For example,
do we seek information related to the sample transmitted
to the laboratory for test or do we need information
concerning the batch from which is prepared the sample ?
It is obvious that the uncertainty will be different in both
cases.
Presentation of the four approaches
The “modelling approach”
The GUM procedure of evaluation of uncertainty is
described in chapter 8 of the GUM. This procedure is based
on a model (mathematical model) formulated to account
for the interrelation of the input quantities that influence
the measurand. Correction is included in the model to
account for systematic effects; such corrections are
essential to achieve Traceability to stated references (e.g.
CRMs, reference measurement procedure, SI units). The
application of the propagation law of uncertainty enables
to evaluate the combined uncertainty on the result. The
model is often expressed in the form of an equation called
“mathematical model” in the GUM.
The “single laboratory validation approach”
When the modelization of the measuring process may be
infeasible for economic or other reasons. In such case
alternative approaches may be used. In particular, the
major sources of variability can often be assessed by
The approaches for measurement uncertainties evaluation