The measurand is not the value or quantity that is being measured. In fact, the measurand cannot be described by a numerical value and is therefore not suitable to be subjected to statistical evaluation. Definition of the measurand must be performed using the expert opinion of those professionals with most experience and knowledge of the process as a whole.
Although mathematics is not required, the first step to producing a complete budget for MU is definition of the measurand. It sets the scene for the remainder of the evaluation. Lack of information contained within the description of the measurand propagates an intrinsic uncertainty throughout the uncertainty calculation. The recommended approach is to provide as complete a definition as possible. However, a potentially infinite amount of information can be applied to the definition of the measurand and a complete description may never be possible. Consequently, the approach is to define each measurand to the extent that any further information (or lack thereof) will not significantly impact on the uncertainty quantification, as determined by expert professional opinion.
The complete (or as complete as possible) definition of the measurand ensures transferability of results between different centres using the same parameters to define the measurand, and to differentiate those that are defined differently.