‘identification’ for the first time in econometrics, and gave the now familiar rank and
order conditions for the identification of a single equation in a system of simultaneous
linear equations. The solution of the identification problem by Koopmans (1949) and
Koopmans, Rubin and Leipnik (1950), was obtained in the case where there are a priori
linear restrictions on the structural parameters. They derived rank and order conditions
for identifiability of a single equation from a complete system of equations without reference
to how the variables of the model are classified as endogenous or exogenous. Other
solutions to the identification problem, also allowing for restrictions on the elements of
the variance-covariance matrix of the structural disturbances, were later offered byWegge
(1965) and Fisher (1966).