SEMAINE D’ÉTUDE SUR LE ROLE DE L’ANALYSE ECONOMETRIOUE ETc. 145 
where A. B and z, are the same as in (18), while (152) 
58) 
y" =E(v,IR z)=R 
is the vector of conditional expectations of the current endo- 
genous variables as given by the reduced form, which as before 
ts assumed to be given by (21)-(22) with (23), to repeat 
(50) 
B - 
Then the following relation can be established, 
(60) 
E(y ly, z)=A vy. +B ¢ 
showing in conjunction with (58) that the BEID-system is bi- 
expectational in the sense of 1.2 (3). 
As to the proof of (60), we note that the model becomes 
deterministic if we respecify the primary form (57) and the 
reduced form (59) by deleting all residuals and in the left-hand 
members substitute y; and y, for y, and y,. This follows as 
an immediate corollary from the substitution theorem (53)-(54), 
allowing z to be a vector variable. 
In the debate on « simultaneous equation systems » it has 
been a key point what causal interpretation, if any, can be 
given to the parameters a; of the behavioural equations in the 
primary form (18) of ID-systems (again, see footnote 6). The 
parameters a;, being numerically the same as in the correspond- 
ing BEID-system, relations (57) and (60) give the answer that 
the parameters allow the same cause-effect interpretation as 
in CC-systems. except that whenever a current endogenous 
(59) In the manuscript as presented at the Study Week, the matrix R 
was missing in E(y, | R z,) in formulas (23) and (58). Cf. the paper (b) 
referred to in the subseauent discussion. page 6. footnote (1 
2] Wold - pag. 31