1 The difference between answering a ‘why’-question and answering a ‘how much’-question Marcel Boumans Abstract Generally, experiments and simulations are carried out to answer specific questions. The assessment of the reliability of the answers depends on the kind of question that is investigated. The answer to a ‘why’-question is an explanation. The premises of an explanation have to include invariant relationships and thus the reliability of such an answer depends on whether the domain of invariance of the relevant relationships covers the domain of the question. The answer to a ‘how much’-question is a measurement. A measurement is reliable when it is an output of a calibrated measuring instrument. So, depending on the kind of questions to be answered, a model should fulfill different requirements. Heinrich Hertz distinguished three kinds of requirements: consistency, correctness and appropriateness. Correctness refers to the requirement that the model should contain equations that are representations of the laws of the phenomena to be investigated. This aim for correct models led in economics to the construction of models that were as large and detailed as possible. Simulations carried out on these correct models showed that the output displayed similar characteristics as the phenomenon being studied, which gave a strong support to this kind of modeling. However, this requirement of correctness is not requisite for answering ‘how-much’ questions. As a result the ways models are built to produce reliable results differ for both kinds of questions. In economics, the approach to build correct models is the so-called Cowles Commission program and dominated econometrics till the 1980s. Doubts whether the obtained model equations are invariant with respect to policy interventions, most expressly worded by Robert Lucas, led to alternative programs of which Lucas’s was most influential. In this so-called general-equilibrium approach, models are calibrated. Calibration entails that models are constructed and assessed according a system engineering approach of assembling black boxes of which the outputs displays the desired operating characteristics. Department of Economics University of Amsterdam Roetersstraat 11 1018 WB Amsterdam The Netherlands m.j.boumans@uva.nl