"Data on gross domestic product (called now Gross Domestic Income) are available from three sources: the Maddison project, which is the only source for the long-run series of national GDPs, going back to 1820s; the World Bank or IMF annual data, going back to 1960; and Penn World Tables, produced periodically at the University of Pennsylvania, going back from their just-released version 8.0 to 1950 . All three sources produce GDP data in PPP (purchasing power parity) terms, which means that they adjust for differences in price levels between the countries. The easiest way to explain it is to say that PPPs try to account for each good and service using the same price for it around the world, so that a mobile phone, a kilo of rice and a haircut would each be valued the same in China as in the United States. Only thus can the real sizes of the economies, and the welfare of people, be truly comparable. These PPP data, in turn, are obtained through a massive worldwide project called the International Comparison Program, which is run every five to 10 years and collects more than 1,000 prices in all countries."

http://blogs.worldbank.org/developmenttalk/end-long-era