Revision of the 'Guide to the Expression of Uncertainty in Measurement'. Why and how

The 'Guide to the Expression of Uncertainty in Measurement' has now served for more than twenty years. In this communication, after attempting a balance over this period, the logical reasons are given that, on the one hand, led to the decision to update such a successful document, and, on the other hand, dictated the modifications that are being carried out with respect to the current 2008 edition. The author is convener of the Joint Committee for Guides in Metrology (JCGM) Working Group 1 (Guide to the expression of uncertainty in measurement, or GUM). The opinion expressed in this paper does not necessarily represent the view of this Working Group.