1. Bureau of the Census (1981; 1991).Statistical abstract of the United States. Washington, DC: U.S. Government Printing Office.
2. Bureau of the Census (1975).Historical statistics of the United States, colonial times to 1970. Washington, DC: U.S. Government Printing Office.
3. Dept. of Energy (1992).Monthly energy review. Washington, DC: U.S. Government Printing Office.
4. Mazur, A. (1991).Global social problems. Englewood Cliffs, NJ: Prentice-Hall.