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PONTIFICIAE ACADEMIAE SCIENTIARVM SCRIPTA VARIA - 2§ 
the reduced form (21) cannot be obtained from the primary 
form (18) by iterated substitutions. This last feature is closely 
related to the fact that ID-systems are not bi-expectational : 
the predictor specification (13) of the primary form (12) of 
CC-systems has no parallel in ID-systems (18). The lack of 
parallel to (13) in (18) is clearly a stochastic feature of the 
models, for it would not appear if the ID-systems (18) were 
deterministic in the sense of disturbance-free relations; in fact, 
the left-hand members of (23) would then be nothing else than 
the component variables y, and (23) would be precisely the 
same system of relations as (21). 
[n the much-discussed dualism between CC- versus ID- 
systems it has been a veritable stumbling block that ID-systems 
are not bi-expectational (%). As briefly noted above, this key 
feature results from the merging of two lines of generalization, 
namely from VR- to CC- and ID-systems on the one hand, 
and from deterministic to stochastic specification of the models 
on the other. We shall return to this matter in section 2 for 
a more detailed review. 
3. Accounting identities vs. equilibrium relations. To sum- 
marize the argument, accounting identities make no incentive 
in the generalization from VR- or CC-systems to’ ID-systems, 
whereas the incorporation of equilibrium relations into the 
model is one of the main incentives in the generalization from 
CC- to ID-systems (7). 
The argument will be illustrated by simple cases in point. 
A typical accounting identity is given by 
a 
Y,=C, +8, 
(°) See, also for further references, Refs. 16 to 18 and Ref. 30. ; 
() The exposition makes systematic use of eo ipso predictors; otherwise 
‘he argument of this subsection is well known. 
2, 
Wold - pag. 14