SEMAINE D ETUDE SUR LE ROLE DE L ANALYSE ECONOMETRIOUE ETC. 416 
ate economy-wide model, there are likely to be such systematic 
effects influencing individual equations and even whole sectors. 
Systematic effects which spread over more than one sector, 
however, seem substantially less likely to occur, especially when 
we recall that the limits of a sector in our sense are likely to 
be rather wide (*?). Variables which give rise to such effects 
are not likely to be omitted variables whose influence lies in 
the disturbance terms. Rather they are likely to be explicitly 
included in the model, if at all possible. If not, if they relate 
to the occurrence of a war, for example, and are thus hard 
to specify explicitly, the time periods in which they ate most 
important are likely to be omitted from the analysis. In short, 
systematic behavior of the disturbances is an indication of 
incomplete specification. Such incompleteness is much less 
likely to occur as regards effects which are widespread than 
as regards effects which are relatively narrowly confined, espe- 
cially since the former are less likely to be made up of many 
small effects (**). (Recall that an economy-wide implicit 
disturbance is one which affects more than one sector directly, 
not simply one whose effects are transmitted through the dy- 
namic causal structure of the explicit model.) It thus does not 
seem unreasonable to assume that: 
(5.18) 
and therefore 
(5.19)  V(0)Y =c 
…, N, 
as good approximations. 
(*¥) As they are in the Brookings-SSRC model. 
(“) A similar argument obviously implies that sector implicit disturban- 
ces are less likely to be serially correlated than are equation implicit disturb- 
ances. The analysis of the effects of this on V(@) and the subsequent 
discussion is left to the reader. The assumption of no serial correlation in 
the sector implicit disturbances seems considerably more dangerous than 
that being discussed in the text. 
6 
Fisher - pag. 35