2015 Rankings are based on the GDP (PPP) of a country, which compares the generalized differences in the cost of living and standards between countries.
A small size, a large endowment of natural resources and/or a highly developed banking system appear to be key ingredients for countries that hope to achieve high standards of living, not necessarily an easy model to replicate. In fact, according to the International Monetary Fund, the ranking of nations by per-capita income is dominated by those that possess such characteristics, starting with Qatar, which, in 2015, has an estimated per-capita GDP (PPP) more than 50% higher than second placed Luxembourg. The United States, which comes in at number nine is the only truly large and diverse economy on the list.
Overall, there are two standard methods of defining the richest countries in the world. One takes into account the economies that are the largest, as measured by total gross domestic product (GDP). However, the most commonly accepted definition of the wealthiest countries is to determine how rich the average resident of a country is. For this reason, the best method is to use GDP data per capita.
Moreover, using a PPP (purchasing power parity) basis is arguably more useful when comparing generalized differences in living standards on the whole between nations. This is because PPP takes into account the relative cost of living and the inflation rates of the countries, rather than using just exchange rates, which may distort the real differences in income. This is the measure most economists prefer when looking at per-capita wealth and when comparing economic strength between countries and living conditions or use of resources across countries.
Values are expressed in current international dollars, reflecting a single year's (the current year) currency exchange rates and PPP adjustments.