Observables can be directly counted or measured, once a definition has been selected. For example, inflation requires an agreement (not commonly accepted, viz. the "Griliches et al. report" on the proper use of index numbers in U.S. social policy) on the selection of basket of goods and services and the way in which the corresponding prices are measured in (at least) two different periods. GDP measurement is the outcome of a process whereby some money flows in the economy are estimated; nonmoney flows (such as ecological damage and social degradation following from the generation of GDP) are commonly disregarded -- but, otherwise, GDP can be counted "objectively". Unemployment, once the definition of an actual labour force and a potential labour force has been made, is a matter of counting. Needless to say that measurement and counting errors are always present. Intelligent economists and policy makers take these into account. NAIRU is statistically derived from a complex estimation process of the operations of an economy, in which process some assumptions about human behaviour are made. Indeed, one should say that forms of human behaviour is imputed (assumed to be present) in the way the model is set up. Take away the imputations and NAIRU stands stark naked. The reason why it is so popular in some circles is that its use justifies attacks on the welfare state and labour rights -- nothing objective about that, and not at all in the same league as the measurement (even when subject to error) of inflation, GDP and unemployment Jesse Vorst