– the intergovernmental organization through which Member States act together
     on matters related to measurement science and measurement standards.
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CIPM and regional key comparisons
In drawing up the CIPM MRA, the CIPM devised the following scheme – sometimes called the "Mickey's ears scheme" – for the organization of key comparisons:

National metrology institute (NMI) participating in CIPM key comparisons
NMI participating in CIPM key comparisons and in regional metrology organization (RMO) key comparisons
NMI participating in RMO key comparisons
NMI participating in ongoing BIPM key comparisons
NMI participating in a bilateral key comparison
International organization signatory to the MRA

The key comparisons are essentially of two types:

  • CIPM key comparisons, of international scope, are carried out by those participants having the highest level of skills in the measurement involved, and are restricted to laboratories of Member States. The CIPM key comparisons deliver "the reference value" for the chosen key quantity;

  • RMO key comparisons, of regional scope, are organized at the scale of a region (though they may include additional participants from other regions) and are open to laboratories of Associates as well as Member States. These key comparisons deliver complementary information without changing the reference value.

The CIPM delegates the task of organizing comparisons to its Consultative Committees (CCs). Each CC chooses the key quantities to be compared in its field of expertise, approves the protocols of the key comparisons, and approves their results before publication in the KCDB. Each CC is composed of the world's most competent laboratories in the field, and it is some or all of these CC members that participate in the CIPM key comparison, which is then generally referred to as a "CC key comparison" since the field of expertise is defined.

The RMOs organize corresponding RMO key comparisons with a number of common participants and with protocols allowing their results to be linked to those of the CC key comparison once these are treated in terms of equivalence (calculation of a key comparison reference value and of degrees of equivalence). The nomenclature used in the KCDB reflects this organization; for instance CCAUV.A-K1 is the CC comparison to which the regional equivalents APMP.AUV.A-K1, EURAMET.AUV.A-K1, etc. are linked.

CC and RMO key comparisons are carried out over a given period of time, and are then analysed and published. Each exercise may be repeated, in part or in full, according to need. However, a different organizational scheme is required in the case of the unique international facilities kept at the BIPM. For these, comparisons are organized as ongoing series of bilateral comparisons between the BIPM and outside laboratories, and continue as long as there is a need. When such a comparison involves a key quantity chosen by a CC, it is called a "BIPM key comparison".

Together, the set of CC key comparisons and BIPM key comparisons forms the ensemble of CIPM key comparisons conducted in the framework of the CIPM MRA.

A BIPM key comparison can (and often does) serve as the central comparison of the Mickey's ears scheme. The results of a BIPM key comparison are interpreted in terms of equivalence, with a key comparison reference value and degrees of equivalence. Since these comparisons are ongoing, there is a continuous accumulation of new data. According to the case (and the decision of the relevant CC), the key comparison reference value may be re-evaluated when new data arrive (as is the case for the International System of Reference for radionuclides) or may remain constant (as is the case for voltage comparisons, where the key comparison reference value is delivered by the BIPM Josephson standard). Beside the role of central key comparison, a BIPM key comparison may also serve as an on-demand exercise whenever waiting for the next full-scale international CC key comparison is impratical. This option is especially valuable when facilities in the laboratories are not simultaneously available but rather are introduced or modified over the years.

CIPM key comparisons is a generic term to designate the real exercises "CC key comparison" and "BIPM key comparison". Because it allows flexibility, the different nature of CC and BIPM key comparisons (full-scale exercise over a finite period of time versus a permanent ongoing series of bilateral comparisons) facilitates the implementation of the CIPM MRA. The CCs can use these tools at their convenience to best fulfil their needs.