“…The dependence of the quantitative behaviour of Samuelson's 1939 model upon the values of the coefficients of the variables deployed in the equations became increasingly seen as problematic. It indicated to Kydland and Prescott (1991: 165), for instance, that "pure theory was not providing sufficient discipline", which brought into the picture the statistics discipline provided by Tjalling Koopmans 5 See Carret (2021) for a detailed account of the mathematical properties of Frisch's 1933 macrodynamic model. At about the same time, Tinbergen had developed a nonlinear macrodynamic model which he used to shed new light on several policy problems: wage changes, government expenditure and its relation to pump-priming, and the regulation of purchasing power.…”