A.2 Establishing Metrological Traceability

A.2.1 Key Considerations

Metrological traceability is established by considering, and then ensuring, the following:

1. The specification of the measurand (quantity to be measured).
2. A documented unbroken chain of calibrations going back to stated and appropriate references (including national or international standards, and intrinsic standards).
3. That measurement uncertainty for each step in the traceability chain is evaluated according to agreed methods.
4. That each step of the chain is performed in accordance with appropriate methods, with the measurement results and with associated, recorded measurement uncertainties.
5. That the laboratories performing one or more steps in the chain supply evidence for their technical competence.

A.2.2 Systematic Measurement Error

The systematic measurement error (sometimes called "bias") of the calibrated equipment is taken into account to disseminate metrological traceability to measurement results in the laboratory. There are several mechanisms available to take into account the systematic measurement errors in the dissemination of metrological traceability.

A.2.3 Use of Measurement Standards

Measurement standards that have reported information from a competent laboratory that includes only a statement of conformity to a specification (omitting the measurement results and associated uncertainties) are sometimes used to disseminate metrological traceability.

This approach, in which the specification limits are imported as the source of uncertainty, is dependent upon:

• The use of an appropriate decision rule to establish conformity.
• The specification limits subsequently being treated in a technically appropriate way in the uncertainty budget.

The technical basis for this approach is that the declared conformance to a specification defines a range of measurement values, within which the true value is expected to lie, at a specified level of confidence. This considers both any bias from the true value and the measurement uncertainty.