SEMAINE D'ÉTUDE SUR LE ROLE DE L’ANALYSE ECONOMETRIQUE ETC. 
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WoLD 
Prof. MAHALANOBIS’ question is important, and my answer is in 
line with Prof. STONE’s views on the integration between forecasting 
and objectives. The notion of targets is closely related to the notion 
of objectives. Reference is also made to the distinction between 
instruments and targets emphasized in the works of Prof. TINBERGEN. 
In my brief comments on descriptive, explanatory and forecasting 
models I was limiting myself to the purely scientific aspects of model 
building. In the analysis of policy problems we come to a fourth 
type of model, policy models. A frequent type of policy model 
specifies different alternatives of policy in terms of objectives and 
targets to be achieved on the one hand, and instruments to be used 
on the other. Speaking generally, a nonscientific element enters in 
the policy model when it comes to the actual choice between the 
different alternatives of pnlicv 
MAHALANOBIS 
Another supplementary question: in meteorology, some experi 
ments have been made to find out whether rain can be influencea by 
artificial means. That would bring in —- I take Professor 
STONE’s point of experimentation. Would such experimentation be 
included in forecasting? I am trying to get Professor WoLD’s views 
clear in mv mind. 
WoLD 
This question points to the very important distinction between 
two ways of gathering knowledge: by controlled experiments and by 
nonexperimental observations. These two ways of getting access to 
knowledge cut through all levels of scientific model building - - de- 
scriptive, explanatory and forecasting models. My brief outline was 
not intended to be complete, and I choose the three areas meteoro. 
‘1] Stone - pag. g7