Quoting from the 1st paragraph of section 1.2 Uncertainty:
"... quantifies the magnitude of any possible discrepancy
between the value obtained from a measurement of a well-defined
quantity and the value which would be obtained at the highest
level reference laboratory for the same quantity."
This fact, I feel, emphasises the importance of interlaboratory
errors (as treated in ISO 5725 pt. 2) and associated uncertainties.
It is true to say that rarely do measurements involve direct
participation in an interlaboratory accuracy experiment (as the
draft mentions in sections 4.3 and in the Gap Analysis). But behind
every measurement is the requirement of being able to relate the
results to those of others (through the use of a coherent measurement
system such as the SI). And uncertainties associated with this
traceability need to be dealt with.
In this connection, the question arises: How does one best tackle
measurement uncertainties w r t trueness? The hypothetical "true
value" can be introduced as follows:
Since relations between measurement results are of the
essence, the concept of measurement compatibility (which is missing
from GUM, for example) is important:
Two measurement results are compatible if their difference
is less than their combined uncertainties.
One measurement result may have much smaller uncertainties than
the other in a comparison - the former then plays the role of
a de facto true value. A traceability hierarchy
can of course then be established where different measurement
results are ranked according to their uncertainties - the so-called
traceability level of each measurement.
In summary, the question of uncertainties in traceable measurements
seems to be tractable by extending the ISO 5725 Accuracy experiments
in combination with GUM.