The Decennial Census occurs every 10 years ending in zero, to count the population and housing units for the entire United States. Its primary purpose is to provide the population counts that determine how seats in the U.S. House of Representatives are apportioned.
Migration data are derived from the census long form question on residence 5 years ago at almost any geographical level. Data are available from 1940 to 2000.
Decennial census tables on geographical mobility/migration are available from American FactFinder for 1990 and 2000.
Tabulations from the most recent and historical censuses are also available from the Census of Population and Housing Publications Web site.