166 PONTIFICIAE ACADEMIAE SCIENTIARVM SCRIPTA VARIA 
be done without forecast errors. The relationships describing 
the impact of these forces and of the variables controlled by 
the decision maker are not known with certainty either. The 
decision maker must therefore make his decisions (prepare his 
Plan) under conditions of uncertainty. Now there is the im- 
portant feature that this uncertainty diminishes in the course 
of time. For example, how the outside forces behave in year I 
is unknown at the beginning of the year, at least not known 
with certainty, so that the decision to be made at the beginning 
of that year must be based on a forecast which is generally 
imperfect. At the beginning of year 2 one should know more 
about how the outside forces have behaved in year I; also, one 
may have better ideas about how they will behave in year 2, 
because this is now « nearer future » than it was a year before. 
Clearly, the decision maker is in a position to take this new 
information into account when formulating his decision for 
year 2. But the Plan does not! The Plan was made at the 
beginning of year 1 and what it has to say about things to be 
done in year 2 is therefore necessarily based on the smaller 
amount of information which was available at the beginning of 
vear I. 
This is of course generally recognized. It is the reason why 
a Plan is almost never taken completely seriously in the sense 
that it is executed literally until the very end. Plans are re- 
vised regularly in the light of new evidence. But this happens 
after the events, not before as is done by a « strategy » or 
« decision rule ». The main argument of this paper is that it 
is worthwhile to consider the possibility of development strate- 
gies instead of development plans. That is, rather than fixing 
once and for all what is to be done in year 1, in year 2, in 
year 3, ..., we consider a procedure of the following kind: 
now, at the beginning of year 1, a decision for that year is 
formulated and at the same time also a rule specifying what 
is to be done in years 2, 3, ... depending on the information 
that will be available by that time. The advantage of this pro- 
71 Theil - pag. 
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