 According to model-based accounts, measurement involves interaction between an object of interest (the “system under measurement”), an instrument (the “measurement system”) and an environment, which includes the measuring subjects. Other, secondary interactions may also be relevant for the determination of a measurement outcome, such as the interaction between the measuring instrument and the reference standards used for its calibration, and the chain of comparisons that trace the reference standard back to primary measurement standards (Mari 2003: 25). Measurement proceeds by representing these interactions with a set of parameters, and assigning values to a subset of those parameters (known as “measurands”) based on the results of the interactions. When measured parameters are numerical they are called “quantities”. Although measurands need not be quantities, a quantitative measurement scenario will be supposed in what follows.