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The fundamental reason for conducting the decennial census of the United States is to determine how the 435 members of the House of Representatives will be apportioned among the 50 states.
Apportionment is based on the size of the resident population for each state plus the state's overseas population of federal employees (military and civilian) and their dependents. Overseas missionaries or employees of multi-national companies are not included.
As populations grow and decline among the states, Congressional apportionments have shifted. For instance, after the Census 2000 enumeration, 8 states gained seats in the House and 10 lost seats.
Our Apportionment Adjustment Calculator allows one to adjust apportionment populations and see what happens to the apportionment counts. You can adjust the population for one, two, or all states and have new apportionment counts selected. Try it out!
The most recent challenge to the apportionment process is a complaint [pdf] about the size of the House of Representatives. Five states claim that their votes carry less weight in the House than others and would like to see the apportionment formula based on a larger base.
In the past, there have been decisions about the use of imputation, Utah v. Evans, 536, 452 (2002), statistical sampling, Department Of Commerce v. United States House (98-404), the use of unadjusted figures, Wisconsin v. City of New York et al. (94-1614), 517 U.S. 1 (1996), the apportionment formula, United States Dep't of Commerce v. Montana (91-860), 503 U.S. 442 (1992), and including the overseas population in the apportionment population, Franklin v. Massassachusetts. (91-1502), 505 U.S. 788 (1992).
The following links offer more information on the apportionment process: