How should metrology bodies treat method-defined measurands?

This discussion article explores how metrology bodies should approach method-defined measurands (also called operationally defined measurands). It begins by considering the different types of measurement that can be encountered and comparing their difference qualities, before discussing in more detail method-defined measurands and the nature of their current and future treatment by the global metrology system. The discussion highlights work performed within the Consultative Committee for Amount of Substance to address what should be within the scope of the International Committee for Weights and Measures Mutual Recognition Arrangement (CIPM-MRA) when method-defined measurands are considered and also how the metrology community can contribute to ensuring stability and comparability of these measurements outside the formal scope of the CIPM-MRA.