Purchasing power parity (PPP) is an economic theory that states that the exchange rate between two countries is equal to the ratio of the currencies' respective purchasing power. Theories that invoke purchasing power parity assume that in some circumstances (for example, as a long-run tendency) it would cost exactly the same number of, for example, US dollars to buy euros and then to use the difference in value to buy a market basket of goods as it would cost to directly purchase the market basket of goods with dollars. A fall in either currency's purchasing power would lead to a proportional decrease in that currency's valuation on the foreign exchange market.
The concept of purchasing power parity allows one to estimate what the exchange rate between two currencies would have to be in order for the exchange to be at par with the purchasing power of the two countries' currencies. Using that PPP rate for hypothetical currency conversions, a given amount of one currency thus has the same purchasing power whether used directly to purchase a market basket of goods or used to convert at the PPP rate to the other currency and then purchase the market basket using that currency. Observed deviations of the exchange rate from purchasing power parity are measured by deviations of the real exchange rate from its PPP value of 1.
PPP exchange rates help costing but exclude profits. So, it is reckoned as more efficient methodology than the use of market exchange rates. For example, suppose that two countries produce the same physical amounts of goods as each other in each of two different years. Since market exchange rates fluctuate substantially, when the GDP of one country measured in its own currency is converted to the other country's currency using market exchange rates, one country might be inferred to have higher real GDP than the other country in one year but lower in the other; both of these inferences would fail to reflect the reality of their relative levels of production. But if one country's GDP is converted into the other country's currency using PPP exchange rates instead of observed market exchange rates, the false inference will not occur. Essentially GDP PPP controls for the different costs of living and price levels, usually relative to the United States Dollar, thus enabling a more accurate depiction of a given nation's level of production.