DISCUSSION 
FISHER 
[ have only a minor comment on Professor ISARD’s paper, it is 
in connection with his use of weights, He uses an expression in 
which W, and Ww, appear as weights; however, there is nothing about 
W, which is specific to i or Ww. which is specific to 4 in that ex- 
pression, since the form of the expression in fact allows these to 
be interchanged and the wrong weights applied to the two items. 
The W, and Ww, therefore ought not to be referred to as weights in 
the strict sense. On the other hand, IsARD uses these in obtaining 
a weighted sum of items, the sum being taken over all pairs i 
and 7. When that is done it is the case that W, appears in every 
pair involving CD, and W, appears in every pair involving CD, 
Thus each pair receives a weight equal to the product of W, and W, 
and it is these products which should be considered as weights. It is 
thus true that W. is specific to ‘ and W *o 4 although only the 
pairs are use’ 
TSARD 
Professor FISHER is correct in the context in which he speaks 
However, in much of the literature using gravity models, 1! Wel us 
W. and W, are used singly, and not ‘» produc form This + ‘he 
reason I do so throuchout mv manuse= 
Isard 
pag. 27