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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 simplest
 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 expected
 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 characteristic
 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 regression
 relations, I have many times voiced scepticism about interdependent
 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 endogenous
 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