NGS prefers to use gross domestic product (GDP)—not gross national income (GNI), the new designation for gross national product (GNP)—to
determine per capita income, and also to convert into dollars using
purchasing power parity (PPP) calculations rather than official currency
GDP gives a more reliable indication of the economy
within a country and PPP a fairer indication of purchasing power and
therefore a better comparison of economic well-being between countries.
Reliable sources for GDP per capita income are the National Geographic Atlas of the World (eighth edition) and the CIA website at http://www.odci.gov/cia/publications/factbook/country.html (go to the "Economy" section of each country listed).