Koopmans did not, however, emphasize the wider issue of the use of stochastic models
in econometrics. It was Haavelmo who exploited the idea to the full, and argued for an
explicit probability approach to the estimation and testing of economic relations. In his
classic paper published as a supplement to Econometrica in 1944, Haavelmo defended the
probability approach on two grounds: firstly, he argued that the use of statistical measures
such as means, standard errors and correlation coefficients for inferential purposes is
justified only if the process generating the data can be cast in terms of a probability
model.