SEMAINE D'ÉTUDE SUR LE ROLE DE L’ANALYSE ECONOMETRIOUE ETC.
we obtain is, let us say, a demand parameter? This problem
1as been the subject of intensive study at the Cowles Commis-
sion [11] [12] and more recently has been given an elegant
form by presentation by Hurwicz [18] and, in terms of sta-
‘istical estimation, by DURBIN [13].
Important as these developments are, they certainly do not
solve all the problems that the model-builder has to face. One
of the main problems arises from the fact that relationships
are constantly changing. I have already given consumers’
preferences as an example of this. Another example is of-
fered by the techniques of production: a past input-output table
will not be a good description of present intermediate techno-
ogy; a fortiori it will not be a good description of the inter-
mediate technology of the future. How then are we to proceed?
The best thing to do is undoubtedly to consult outside experts
about the way in which inputs have been changing, and to
get from them an estimate of future input structures in different
industries. But such information can only be expected from a
limited number of highly articulate industries. In other cases
ne can sometimes find time-series of input-output coefficients
and project these into the future. Often, however, this approach
is closed too, and there is nothing to be done but to project
nput-output coefficients by a general method of extrapolation.
A means of doing this is given in [36] and, in greater detail.
n [9].
Another practical problem arises from the fact that, in
formulating relationships, we usually begin by considering only
the more general influences which we believe to be at work.
For example, in formulating demand relationships we usually
begin by allowing for the effects of income, prices and chang-
ing tastes on the demand for different commodities. We know
of course that other, more specific, influences are at work: an
abnormally cold winter or hot summer, the temporary rationing
of some commodity, a particular advertising campaign. Such
specific influences are numerous and difficult to take into ac-
“Stone - pag.
