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Reilly's law

  A method derived by a market researcher for estimating the relative flow of trade from a place to each of two towns (Reilly, 1931). Built on the same foundations as the gravity model, the law states that \'two cities attract trade from an intermediate town in the vicinity of the breaking point approximately in direct proportion to the populations of the two cities and in inverse proportion to the squares of the distances from these two cities to the intermediate town\'. Algebraically this is represented as:

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where Ta and Tb are the proportions of the trade (T) going to towns a and b respectively, Pa and Pb are the populations of a and b respectively, and da and db are the distances from the place being considered to a and b. All other things being equal, more trade will go to the larger cities and to the closer places. (Note that in central place theory all trade goes to the closest centre, whatever its size.) (RJJ)

Reference Reilly,W.J. 1931: The law of retail gravitation. New York: Knickerbocker Press.



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