CRITERIA FOR THE
SELECTION OF A PROFICIENCY TESTING SCHEME
Background
Participation in
interlaboratory comparisons and proficiency testing (PT) is an important tool
for laboratories to check the reliability of their results by comparison within
their peer group and to demonstrate their performance to clients and
accreditation bodies. With the increasing availability of PT schemes in many
technical fields, the criteria for the selection of an appropriate scheme are
becoming more important.
Since some accreditation
bodies now accredit PT providers, there may be a tendency for either
accreditation bodies or individual assessors to recommend participation in
accredited PT schemes or to require an accredited laboratory to justify its
selection of a non-accredited PT scheme.
The purpose of this
document is to establish the relationship between accreditation and the
criteria for the selection of a PT scheme.
Technical Criteria
In a joint paper of EA,
EUROLAB and EURACHEM on proficiency testing [1] some technical criteria for the
selection of an appropriate PT scheme are given:
- Material/Matrix: The
sample material/matrix is as close as possible to that normally tested by the
laboratory.
- Measurands: The
measurands in the test samples or materials include as many as reasonable of
those normally measured by the laboratory in that sample type.
- Levels: The
levels of these measurands are broadly within the range usually measured by the
laboratory in that sample type.
- Frequency: The
frequency of rounds of the scheme is sufficient as recommended in EA-3/04 [2]
in connection with the other means of quality applied by the laboratory for the
respective test (method, measurand, material/matrix).
- Statistical
protocol: The statistical protocol for evaluating the participants’
performance is considered to be appropriate for the measurands and test methods
covered.
Selection
When selecting a new PT
scheme, a laboratory should focus on the technical criteria. It should be noted
that it might not be possible to participate in or even find PTs for all
methods, materials/matrices, measurands and measurement ranges that the
laboratory carries out. A reasonable and effective way of covering similar
tasks has to be accepted [1]. Thus, a laboratory may have to accept some
compromises because a PT scheme ideally suited to its needs might not be
available.
When a laboratory
participates in a PT scheme for the first time, it should be able to give
reasons for the choice that it makes based on technical criteria, but it is not
necessary for it to have to justify the selection to the extent that, for
example, a customer audit would be required.
After the finalisation
of the PT round, the laboratory should assess not only its own performance in
relation to its peer group of laboratories but also the performance of the PT
provider and the appropriateness of the scheme with regard to its own needs.
For example, the report delivered by the PT provider and any support to take
appropriate corrective actions, if necessary, might be important criteria for
the quality of the PT provider’s services and influence future decisions on the
choice of a PT provider.
For clarity, it is
important to note that a PT provider is a supplier of an external service as
mentioned in section 6.6. In relation to the laboratory, the PT provider is in
a similar position to the producers of laboratory equipment or of consumable
materials. It is not a requirement of ISO/IEC 17025 for a PT provider to be
accredited. But ISO/IEC 17025 gives in section 7.7.2 a) a note that PT provider
that meeting the requirements due to ISO/IEC 17043 are considered to be
competent. So an accreditation of a PT provider does not requires any other
evidence concerning the competence of the PT provider. But a laboratory can
base its decision to participate in a specific PT scheme also on other criteria
as for example on its experience of the scheme in the past. Further guidance on the
selection, use and interpretation of PT schemes can be found in the EURACHEM
document [4].
Conclusions
In choosing an
appropriate PT provider, the laboratory has a number of choices to make.
Accreditation of a PT provider is one criterion. Other technical criteria could
be more relevant.
EUROLAB promotes the
approach that a laboratory should select the most appropriate PT scheme
according to its needs. The laboratory should subsequently assess the
appropriateness of a scheme in the light of the results gained. This also holds
in the case of an accredited PT provider.
References
[1] EA, EUROLAB,
EURACHEM position paper on the “Trade-off” issue between Participation in
Proficiency Testing and
the Level and Frequency of Surveillance Activities, 2004
[2] EA 3/04 (rev01), “Use
of Proficiency Testing for Accreditation in Testing”,
www.european-accreditation.org
[3] ISO/IEC 17025:2017,
“General requirements for the competence of testing and calibration
laboratories”
[4] EURACHEM Guide on “Selection,
use and interpretation of proficiency testing (PT) schemes by laboratories”,
2000,
www.eurachem.org
