Edward Ullman

Edward Ullman (1912 – 1976) was an American geographer. He proposed that trade was an interaction based on three phenomena: complementarity, intervening opportunities, and transferrability of commodities.

The level of interaction can be measured by the Gravity model of trade:

$$ I_{i,j} = \frac{p_ip_j}{d_{i,j}^\beta} $$

Where:
 * I: Level of interaction between i,j. alternatively, quantity of trade between i,j.
 * Pi = population of i
 * Di,j = distance separating i,j
 * β = impedance factor