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PONTIFICIAE ACADEMIAE SCIENTIARVM SCRIPTA VARIA - 28 
Further, it is evident that all full-information methods are 
rather sensitive to specification errors of the types that are 
unavoidable in the foreseeable state of econometric models 
which are only approximate. In particular, such estimators 
have the defects of their merits in that by using information 
on the entire system to estimate any single equation, they carry 
the effects of specification error in any part of the system to 
the estimate of any other part. Since it is clear that some 
equations may be thought to be better specified than others as 
the quality of economic information is by no means constant 
in an economy-wide model, this is a highly undesirable feature. 
It seems clear that specification error should be quarantined and 
hence that limited-information estimators which are known 
to accomplish this are preferable to full-information ones in 
large models (*). While it may be desirable to use intermediate 
estimators which apply full-information type methods to sectors. 
rather than to the system as a whole, -the theory of how this 
should be done remains to be worked out. 
4. LIMITED-INFORMATION ESTIMATORS 
4.X. Availability in Practice 
While a great many limited-information estimators have 
been suggested, in practice, none of them are available for use 
in their original form in estimating an economy-wide econome- 
tric model. This is the case because all such estimators begin 
in one way or another with the ordinary least squares estimates 
of the reduced form equations. As we have already seen, such 
estimates are likely to be difficult or impossible to secure in all 
but the most aggregate models because of the low number of 
(1) Fisuer [8, p. 155] 
61 Fisher - pag. 18