What is measurement uncertainty? A discussion

In 1993, the Guide to the Expression of Uncertainty in Measurement (GUM) defined measurement uncertainty as a parameter, and therefore a mathematical entity. In the following 30 years this definition has been maintained as a reference, although it has increasingly proven to be too narrow in its scope. In 2023, the Introduction to GUM introduced a different definition, which implied three radical changes to the well-established and widely adopted conceptual framework of the GUM, as adopted by the International Vocabulary of Metrology (VIM). These involved treating measurement uncertainty as (i) a psychological entity instead of a mathematical one, (ii) related to the doubt about a theoretical true value instead of to an operational dispersion of measured values, and (iii) only applicable to single-valued measurands instead of the more general and more useful case of measurands with non-zero definitional uncertainty. Since the same changes are now proposed for inclusion in the next edition of the VIM it is appropriate to carefully analyze them. Our conclusion is that there are no sufficient reasons for radically changing the fundamental concept of measurement uncertainty. Instead, the definition of the GUM should rather be generalized to make it more widely applicable.