SEMAINE D'ÉTUDE SUR LE ROLE DE L ANALYSE ECONOMETRIOUE ETC. 
Third, the programme can be subdivided into independent 
parts each of which can be contained, data and programme, 
in the high-speed memory of the computer. This allows each 
stage to be developed and tested independently so that, as 
research progresses, it can be replaced by more complicated 
versions without necessarily upsetting the remaining stages. 
Fourth, the programme can be used to compile ‘ready 
reckoners’ which make it possible to trace the effect of changes 
n the initial conditions without the need for further runs. For 
example, the matrix multiplier (I-A)-! is a ready reckoner 
whose elements show the additional output of ÿ which will 
‘ollow directly or indirectly from a unit increase in the final 
demand for k. Analogous matrices can be compiled to show 
the effect of unit changes in the elements of final demand on 
complementary or competitive imports. on investment require- 
ments, and so on. 
Finally, once an initial model has been set up, it is necessary 
to establish an order of priorities in improving its various parts. 
Sor this purpose, sensitivity analysis, that is the systematic 
>xploration of the model to find out what is sensitive to what. 
would probably be very useful. 
POLICIES 
Policies express what we want from a system after we have 
reconciled competing objectives in the light of a model; they 
provide therefore a set of consistent aims. As I have said, the 
‘ormulation of objectives is primarily the responsibility of the 
politician, just as the construction of models is primarily the 
responsibility of the scientist. But the politician, like the 
scientist, cannot perform his task unaided and simply hand 
down a set of objectives to the model-builder without discussion. 
Three reasons for this have been suggested by HrrcH in [17]. 
One is that there is seldom a clear-cut national objective whic! 
"11 Stone - pag. TC