Models as measuring instruments

Nancy Cartwright views models as blueprints for nomological machines - machines which, if properly shielded, generate lawlike behaviour or regularities. Marcel Boumans has argued that we can look for devices inside models which enable us to measure aspects of these regularities. So, if models do produce regular behaviour (Cartwright), they might perhaps generate numbers about phenomena in the world, provided we can locate a good measuring device in the model (Boumans). How do they do this? This paper suggests that models can be understood to consist of internal principles and bridge principles, and that - provided the models is calibrated - this suffice for measurement. This paper explores this construction of a measuring device in models by analysing the measurement of duration dependence of unemployment. Since search theory can’t be made operational for this purpose duration models which model of the outflow process of unemployment with a Weibull function are taken as a representation of reservation wage setting in search models. As I show, this Weibull function serves as a bridge principle to make a measure of the elasticity of response between unemployment benefit and duration of unemployment. It enables measurement to take place by modelling behaviour according to some assumptions, which operate as constraints. While this Weibull function serves this purpose, its assumptions are arbitrary rather than connected to theory, and measurements generated with this bridge principle turn out to be highly sensitive to its specification.