1. The history of science and technology;Bunch,2004
2. U.S. Census Bureau, Historical Estimates of World Population (Updated 30 April 2004; http://www.census.gov/ipc/www/worldhis.html).
3. U.S. Census Bureau, Total Midyear Population for the World: 1950–2050 (Updated 30 April 2004; http://www.census.gov/ipc/www/worldpop.html).
4. Macroeconomics;DeLong,2002
5. National Center for Education Statistics, Digest of Education Statistics (2003; http://nces.ed.gov/programs/digest/d03/tables/dt030.asp).