Edward Ullman

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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

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