... We describe one detailed base level model, a component model for Quantity, which is a generalisation of previous informal proposals. ... In Figure 1 we propose a model for this area. The main difference between this and other proposals (see http://www.ivoa.net/twiki/bin/view/IVOA/IVOADMQuantityWP for IVOA Quantity data modeling resources ) is that we believe that the core concept is the measurement , which we define as the act of assigning a value plus error to a property, not the quantity itself. ...