The purchasing power parity (PPP) theory uses the long-term equilibrium exchange rate of two currencies to equalize their purchasing power. Developed by Gustav Cassel in 1920, it is based on the law of one price: the theory states that, in an ideally efficient market, identical goods should have only one price.
This purchasing power exchange rate equalizes the purchasing power of different currencies in their home countries for a given basket of goods. Using a PPP basis is arguably more useful when comparing differences in living standards on the whole between nations because PPP takes into account the relative cost of living and the inflation rates of different countries, rather than just a nominal gross domestic product (GDP) comparison. The best-known and most-used purchasing power parity exchange rate is the Geary-Khamis dollar (the "international dollar").
PPP exchange rates (the "real exchange rate") fluctuations are mostly due to market exchange rates movements. Aside from this volatility, consistent deviations of the market and PPP exchange rates are observed, for example (market exchange rate) prices of non-traded goods and services are usually lower where incomes are lower. (A U.S. dollar exchanged and spent in India will buy more haircuts than a dollar spent in the United States). PPP takes into account this lower cost of living and adjusts for it as though all income was spent locally. In other words, PPP is the amount of a certain basket of basic goods which can be bought in the given country with the money it produces.
There can be marked differences between PPP and market exchange rates. [1] For example, the World Bank's World Development Indicators 2005 estimated that in 2003, one United States dollar was equivalent to about 1.8 Chinese yuan by ...