PONTIFICIAE ACADEMIAE SCIENTIARVM SCRIPTA VARIA 
which, being controlled by the economy and by the plan, 
would correct the instructions given by the model, as illustrated 
in diagram 2 below. 
DIAGRAM 2 
A Corrective Device for Economic Models 
A familiar, if somewhat formal, example can be given to 
illustrate this scheme. Suppose that the purpose of the model is 
to tell us how much to produce of a certain commodity and, 
under free market conditions, what price to charge. Suppose, 
further, that market conditions are not always free but that at 
certain times the government imposes price control. If we 
set up the model in terms of a demand relationship and a supply 
relationship, we may assume for purposes of argument that: 
1] Stone - pag. 16