In order to evaluate economic policy, models are built and used to produce numbers to inform us about economic phenomena. Although phenomena are investigated by using observed data, they themselves are in general not directly observable. To ‘see’ them we need instruments, and to obtain numerical facts about the phenomena in particular we need measuring instruments. This view is a result of James Woodward’s (1989, see also Bogen and Woodward 1988) account on the distinction between phenomena and data. According to Woodward, phenomena are relatively stable and general features of the world and therefore suited as objects of explanation and prediction. Data, that is, the observations playing the role of evidence for claims about phenomena, on the other hand involve observational mistakes, are idiosyncratic and reflect the operation of many different causal factors and are therefore unsuited for any systematic and generalising treatment. Theories are not about observations – particulars – but about phenomena – universals.