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	Urban Primacy Index	Gravity Model of Migration
Field	Human Geography	Human Geography
Family	Process / pipeline	Regression model
Year of origin	1939	1946
Originator	Mark Jefferson	George Kingsley Zipf (formalized); analogy to Newton's law of gravitation
Type	Index of the dominance of the largest city in an urban system	Spatial-interaction regression model for migration flows
Seminal source	Jefferson, M. (1939). The Law of the Primate City. Geographical Review, 29(2), 226–232.	Zipf, G. K. (1946). The P1 P2 / D hypothesis: On the intercity movement of persons. American Sociological Review, 11(6), 677–686.
Aliases	Primacy Index, Primate City Index, Two-City Primacy Index, Four-City Primacy Index	Migration Gravity Model, Demographic Gravity Model, Zipf P1P2/D Model, Gravity Model of Spatial Interaction (Migration)
Related	4	4
Summary	The urban primacy index measures how dominant a country's largest city is relative to the cities below it in the size hierarchy. It grows out of Mark Jefferson's 1939 law of the primate city, which observed that many countries are headed by a single city far larger and more important than any other. The simplest two-city index divides the largest city's population by the second-largest's, while the four-city index compares the leading city with the combined size of the next three, giving a compact gauge of urban concentration.	The gravity model of migration explains the volume of movement between two places as proportional to the product of their populations (masses) and inversely proportional to the distance separating them, by direct analogy to Newton's law of universal gravitation. Formalized for intercity movement by George Kingsley Zipf in 1946 and embedded in regional science by Walter Isard, it is the workhorse model of human geography for predicting migration, commuting, and other spatial-interaction flows.