The International Comparison Program (ICP) is a worldwide statistical partnership to collect comparative price data and compile detailed expenditure values of countries’ gross domestic products (GDP), and to estimate purchasing power parities (PPPs) of the world’s economies. Using PPPs instead of market exchange rates to convert currencies makes it possible to compare the output of economies and the welfare of their inhabitants in real terms (that is, controlling for differences in price levels).

http://siteresources.worldbank.org/ICPEXT/Resources/ICP_2011.html