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12
“Uncertainty-noise” Le Mans
Acoustique
&
Techniques n° 40
Key features of the CEN guidance
The guidance is … only “guidance” i.e. standardizers may
not follow it. However, it is clear that no one can really go
round it nowadays and the issue can no longer be ignored by
standard makers. It stresses that, although efforts must be
made to take uncertainty aboard measurement standards, this
must keep within the limits allowed by available resources and
without compromising the need to deliver European standards
as quickly as possible.
It stresses very important and very true facts that all parties
concerned do not have necessarily in mind:
- The negative aspect behind the word “uncertainty”, which
makes everyone think of faults, mistakes and errors should
be fought against. Uncertainty should be seen positively as it
implies confidence in the validity of results.
- Making decisions without taking account of the uncertainty is
only apparently easy. Serious mistakes may be made resulting
in disputes and adverse consequences.
- Uncertainty is a preventive tool that will avoid in the end
unpleasant surprises.
It provides a useful and easy checklist to assess the need for
implementation of the uncertainty of measurement concept
in standards and guides through the decision process. It also
provides a list of reference standards and documents.
It is publicly available on CEN Website www.cenorm.be/boss.
■
The CEN guidance :
Version 1 of September 2004
Guidance - Uncertainty of measurement concept in European Standards
Source: Resolution BT 21/2003
The CEN guidance as taken from CEN/BOSS is repeated hereafter word for word.
1 Introduction and definitions
When possible and necessary, the Technical Board of CEN (BT) asks CEN technical bodies and other relevant bodies to
implement the guidance and recommendations about uncertainty of measurement (see below) within the limits allowed by
available resources and without compromising the need to deliver European Standards as quickly as possible (resolution BT
21/2003).
Consequently, this document gives guidance to all technical bodies in charge of drafting standards. It provides a checklist to
assess the need for implementation of the uncertainty of measurement concept in standards and guide them through their
decision process.
1.1-Background
It is of common knowledge that measurement results are never perfectly accurate. In practice the sources of systematic and
random errors which can affect the results of measurement are numerous (even for the most careful operators).
To describe this lack of perfection, the term «uncertainty» is used. Although the concept of uncertainty may be related to a
«doubt», in the real sense the knowledge of uncertainty implies increased confidence in the validity of results.
The figure below illustrates the decision difficulty when uncertainty could affect the compliance of a product to a specification
limit (this figure illustrates the case of a Gaussian distribution, but the difficulty is the same in the case of all other kinds of
distribution).
This margin of doubt should be quantified to be able to
make a consistent decision. In fact a key consideration
is the degree of risk associated with the decision making
process.
Without knowledge of the accuracy (trueness and
precision) of measurement methods and/or the
uncertainty of measurement results, it may appear
very easy to make decisions. But, in practice, these
decisions may be incorrect and sometimes lead to
serious consequences, if the measurement uncertainty
is not taken into account.
For example, in the economical field, when rejecting instead of accepting a good product during a certification process or,
conversely, when accepting a bad product by error. In the legal field, when returning a verdict of guilty instead of not guilty
in case of market surveillance or of accident. In the human field, when falsely classifying dangerous products as safe. In the
ethical field, when having overly optimistic or unduly pessimistic interpretation of results leading to a non - fair competition
between manufacturers and between testing laboratories, etc.
So, it is vital to quantify the reliability of the measurement results to greatly reduce any disputes and adverse consequences
of legal proceedings. This is of particular importance if we consider the growing number of cases of litigation in Europe and
the liability problems of manufacturers in case of accidents.
Dealing with uncertainties in european standards