The current table includes 188 countries ranked by their Gross Domestic Product (GDP) as of 2016. The data comes from the latest release of the IMF's World Economic Outlook Database published in October 2016.
According to the definition given in the System of National Accounts 2008 (2008 SNA) - the international statistical standard for the national accounts - GDP is the total value of the final uses of goods and services measured at purchasers' prices over a particular period of time. Thus, it can be said, that GDP represents the size a country's whole economy. In this relation, it can be used to compare countries by the size of their economies. As per this comparison, it appears that the United States had the biggest economy in the world in 2016 with GDP of 18,562 billion U.S. dollars.
Still, using GDP measured in U.S. dollars for international comparisons is not quite correct, because it is affected by the differences in price levels between countries. Purchasing Power Parities (PPPs) are used to remove this effect and to produce comparable estimates of GDP measured in a common currency. According to this measure, China, not the US, is the biggest economy in the world as on 2016.
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