SEMAINE D'ÉTUDE SUR LE ROLE DE L’ANALYSE ECONOMETRIQUE ETC. 1235 
3. The first possibility for simplification consists, of course, 
of a limitation of the number of sectors and of regions. This 
means that the model we are going to discuss must be seen 
as one indicating the main links only; one, therefore, of a 
first orientation and a stage only in the complete process of 
planning. From what follows it will be seen that we will 
listinguish nevertheless between a few categories of sectors, 
meaning that we have in mind a number of around ten sectors 
still, maybe a few more even. As for the number of regions 
anything between two and five may do. This implies that we 
do not propose to deal with the micro-economics of regional 
policy. It remains conceivable, however, that once models 
will be manageable consisting of much larger numbers of sectors 
and regions. For the time being the precise delimitation of the 
regions will not constitute too much of a problem; it is self- 
evident that as much homogeneity in well-being and sectoral 
composition or demographic characteristics as possible must 
be striven for; a further criterion will be that within a region 
transportation costs must be clearly lower than between regions. 
[mportant natural barriers to transportation therefore represent 
natural frontiers between regions. Even with the small number 
of regions proposed meaningful problems may be approached; 
well-known examples of two-region problems are those of Italy 
or of Belgium; examples of three-region countries are Peru 
and Ecuador; problems of urbanization can be dealt with by 
the assumption of « regions » consisting of groups of cities of 
various size classes and a rural region and so on. We must, 
however, reserve one region of our model to represent « foreign 
countries » or the « rest of the world » in order to deal with 
‘he phenomena of imports and exports. That then means that 
in the case of Italy or Belgium, just quoted, we have three 
« regions » and in those of Peru and Ecuador four. We need 
not emphasize that for problems concerning individual giant 
projects a subdivision « ad hoc » of the economy can be 
chosen. 
+, Tinbergen - pag. 3