20) 
PONTIFICIAE ACADEMIAE SCIENTIARVM SCRIPTA VARIA - -§ 
the two prices will, in general, again diverge unless A=o0 is 
changed back to A=1. 
Thus the purpose of the black box is to ensure that the 
model stays in touch with reality. As I have said, the example 
is a formal one, but there is no necessity in practice to restrict 
the device marked B to a single, instantaneous decision. 
d) Computability. With a large model, computability pre- 
sents a problem at every stage: in the preliminary processing 
of data, in estimating the parameters and in reaching solutions 
for the system of equations. I have already emphasised the 
importance of making a start and later modifying and extend- 
ing the initial model; if to this is added the need to reach solu- 
tions for a wide range of initial assumptions, it is clear that 
some special steps will have to be taken if the computing 
problems are to be kept under control. As explained in greater 
detail in [7] [44], a number of straightforward techniques are 
useful for this purpose. 
First, the extensive use of matrix algebra, in addition to 
its notational convenience, has the advantage that in the nu- 
merical manipulation of matrices a fundamentally simple set 
of operations is repeated a fixed number of times. This number 
depends on the order of the matrices and can be regarded as 
a parameter of the programme. Accordingly, if the number of 
categories in some classification has to be increased, it is only 
necessary to change a parameter and continue with the old 
programme. 
Second, great convenience lies in the computer’s facility 
for handling iterative or relaxation processes. By requiring a 
sequence of operations to be repeated until some numerical 
condition is satisfied, the same master programme can be retain- 
ed when linear relationships, which would themselves admit of 
direct analytical solution, are replaced by more complex rela- 
tionships which would not. To do this, a general method of 
solution must be adopted at the outset. 
‘1] Stone - pag. 18