1030 PONTIFICIAE ACADEMIAE SCIENTIARVM SCRIPTA VARIA - 28 
HAAVELMO 
I would first like to say that I am sure Prof. ISARD is right in 
underlining the importance of the kind of problem here mentioned 
and that also very little has been done about it. Now, the remark 
I have refers to Prof. IsarD’s special model of interaction, and is 
based on a recent experience I have had. One of my students came 
to me because he was engaged in some study of retail trade in a 
regional network of shopping centers. Some of these centers were 
small and some larger, and the problem was how to locate stores, 
parking space etc. We got into discussion about the following pro- 
blem. Consider the interaction formula that you have. In my case 
it can be regarded as representing how many customers are drawn 
from 7 to j and vice versa. We found that the symmetry of this 
interaction formula was not adequate. We have a passive element, 
so to speak, and have to distinguish between the active pull and 
the response. We did not find the final answer, but I think it is 
a general problem relevant also to your model. 
[SARD 
I agree that many, if not most situations in the real world involve 
asymmetry. Many suggestions have been made about what the 
measure could be in such cases. For any originating node (region) 
the measure will differ according to type of receiving or terminating 
point (node). Also, I have presented in my paper a regular hierarchy. 
Actually, we know that nodes are differently distributed, and that 
population is of different density in different regions and at different 
nodes. But to introduce an irregular hierarchy causes all kinds of 
problems. I do not have the time now to spell out some of the 
interesting thinking on these problems. 
‘127 Isard - pag. 28