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A 
retain the merit that every instrumental variable used is either 
explicitly included in the equation or contributes significantly 
to the causal explanation of at least one variable so included. 
In practice, there may not be a great deal of difference between 
these alternatives and the last one described may then be 
optimal (unless it is unavailable because of the degrees of 
freedom required). 
Whatever variant of our procedures is thought best in prac- 
tice, they all have the merit of using information on the dyna- 
mic and causal structure of the model in securing estimates. 
The use of such information in some way is vital in the estim- 
ation of economy-wide econometric models where the ideal 
conditions for which most estimators are designed are unlikely 
to be encountered in practice (%). 
(*) The use of the causal structure of the model itself to choose instru- 
mental variables as described in the text is closely akin to the methods used 
by BARGER and KLEIN to estimate a system with a triangular matrix of coef- 
ficients of current endogennus variables See [23] 
61 Fisher - pag. 60