It was R. Frisch, who in his publications 'Correlation and Scatter
Analysis in Statistical Variables' (1929) and 'Statistical Confluence
Analysis by means of Complete Regression Systems' (1934) first pointed
out the complications that arise if one applies regression analysis to
variables among which several independent linear relations exist. Should
these relationships be exact, then there exist two closely related
solutions for this problem, viz. 1. The estimation of 'stable' linear
combinations of coefficients, the so-called estimable functions. 2. The
dropping of the wen-known condition of unbiasedness of the estimators.
This leads to minimum variance minimum bias estimators. This last
solution is generalised in this book for the case of a model consisting
of several equations. In econometrics however, the relations among
variables are nearly always approximately linear so that one cannot
apply one of the solutions mentioned above, because in that case the
matrices used in these methods are, although ill-conditioned, always of
full rank. Approximating these matrices by good-conditioned ones of the
desired rank, it is possible to apply these estimation methods. In order
to get an insight in the consequences of this approximation a simulation
study has been carried out for a two-equation model. Two Stage Least
Squares estimators and estimators found with the aid of the above
mentioned estimation method have been compared. The results of this
study seem to be favourable for this new method.