This contribution describes a Decision Tree intended to guide the
selection of statistical models and data reduction procedures in key
comparisons (KCs). The Decision Tree addresses a specific need of
the Inorganic Analysis Working Group (IAWG) of the Consultative
Committee (CC) for Amount of Substance, Metrology in Chemistry and
Biology (CCQM), of the International Committee for Weights and
Measures (CIPM), and it is likely to address similar needs of other
working groups and consultative committees.
Because the portfolio of KCs previously organized by the CCQM-IAWG
affords a full range of opportunities to demonstrate the
capabilities of the Decision Tree, the majority of the illustrative
examples of application of the Decision Tree are from this working
group. However, the Decision Tree is widely applicable in other
areas of metrology, as illustrated in examples of application to
measurements of radionuclides and of the efficiency of a thermistor
power sensor.
The Decision Tree is intended for use after choices will have been
made about the measurement results that qualify for inclusion in the
calculation of the key comparison reference value (KCRV), and about
the measurement results for which degrees of equivalence should be
produced. Both these choices should be based on substantive
considerations, not on purely statistical criteria. However, the
Decision Tree does not require that the measurement results selected
for either purpose be mutually consistent.
The Decision Tree should be used as a guide, not as the sole and
autonomous determinant of the model that should be selected for the
measurement results obtained in a KC, or of the procedure that
should be employed to reduce these results. The scientists running
the KCs ultimately have the freedom and responsibility to make the
corresponding choices that they deem most appropriate and that best
fit the purpose of each KC.
The Decision Tree involves three statistical tests, and comprises
five terminal leaves, which correspond to as many alternative ways
in which the KCRV, its associated uncertainty, and the degrees of
equivalence (DoEs) may be computed.
This contribution does not purport to suggest that any of the KCRVs,
associated uncertainties, or DoEs, presented in previously approved
final reports issued by working groups of the CCs should be
modified. Neither do the alternative results question existing,
demonstrated calibration and measurement capabilities (CMCs), nor do
they support any new CMCs.
Development of International Key Comparisons in the Field of Chemico-Analytical Measurements
