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: 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