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PONTIFICIAE ACADEMIAE SCIENTIARVM SCRIPTA VARIA - -~ 
2) Parameter 3 is a demand elasticity, an interest rate, or some 
other parameter that typically enters as a coefficient for one of 
the explanatory variables in an economic relationship. In the sim- 
plest case we have 
‘E) 
y =x. 
It is unclear to me what Professor HAAVELMO means when he says 
that B can be an essential element of invariance rather than the ex- 
pected value of y for given x, for what could the right hand member 
of (FE) be assumed to give if not just the expected value of y for 
given x? 
And in the last two lines of his comment I am afraid Professor 
HAAVELMO is not only unclear, but actually mistaken. It is easy 
to give examples where (E) gives the expected value of y for given 
x, and the relation is an invariant that does,not depend on the form 
of the probability laws P and P*. It is even so that this kind of 
invariance has been exploited for assessing the direction of a causal 
relationship (a first approach of this type was initiated by-H. Work- 
ING In 1034; see Ref. 12, section 6). 
3) In the theory of interdependent systems (A) it is a charac- 
teristic feature that the behavioural relations are dealt with as being 
reversible with regard to the current endogenous variables y. This 
feature being in sharp contrast to the irreversibility of ordinary re- 
gression relations, I have many times voiced scepticism about inter- 
dependent systems on this basis (see e.g. Refs. 23, 34). Now the 
respecification (B,) goes some way to clarify the situation. The 
reversibility at issue requires that if we rearrange the current endo- 
genous variables by shifting two or more of them from the one side 
of the relations to the other, then the rearranged system should 
satisfy the corresponding relations of type (B,). On the classic as- 
F217 Wold - pag. 68