SEMAINE D'ÉTUDE SUR LE ROLE DE L’ANALYSE ECONOMETRIQUE ETC. 
Lis 
insists that «it would be useful to attach a time period for public 
investments ». Of course it would, and I fail to see why he feels 
it so necessary to insist on this. Every consequence of a public 
investment occurs at some date. The importance of that conse- 
quence naturally depends on the date or dates at which it occurs. 
The time period and time sequence is incorporated in the analysis 
by means of the shadow prices which are different for every date at 
which something can happen. An event which occurs 10 years 
hence will receive less weight in an analysis than one that occurs 
5 years hence if the shadow prices for events 10 years in the future 
are lower than those for events 5 years in the future, and this wil 
be the case whenever there is a positive rate of time discount. 
SCHNEIDER 
When we consider an economic system which is mainly based 
on the principles of a market economy with a public sector, then 
I don’t see that the problem of shadow prices has so much ‘m- 
portance as it has for example in developing countrie- « ‘- - 
centrallv planned economic swvstem 
DORFMAN 
It is very helpful of you to remind us that where we have marke: 
prices to guide us we have much less need for shadow prices. In 
fact, we may have no need at all. But when public investments 
are being considered, we do not have market prices for all the goods 
and services that concern us. In particular we do not have market 
prices for events that will occur in the future or for the external 
economies and diseconomies that will result from the contemplated 
investment. 
Shadow prices are required for all the consequences of an invest 
ment undertaking that are not valued in the ordinary markets 
y 
Dorfman - pag. 3;