Purchasing power parity (1916)

With its roots in 17th century mercantilism, purchasing power parity was developed by Swedish economist Karl Gustav Cassel (1866-1945). It asserts that exchange rates are in equilibrium when the domestic purchasing power of currencies are the same.

A FFrlO = £1 rate would be in equilibrium if FFrlO bought the same quantity of goods and services in France as £1 bought in Britain. This is a useful concept when comparing international living standards.

Also see: fundamental disequilibrium, internal and external balance

G Cassel, “The Present Situation of the Foreign Exchanges – I’, Economic Journal, vol. XXVI (March, 1916), 62-65


Purchasing power parity is an economic term for measuring prices at different locations. It is based on the law of one price, which says that, if there are no transaction costs nor trade barriers for a particular good, then the price for that good should be the same at every location.[1] Ideally, a computer in New York and in Hong Kong should have the same price. If its price is 500 US dollars in New York and the same computer costs 2000 HK dollars in Hong Kong, PPP theory says the exchange rate should be 4 HK dollars for every 1 US dollar.

Poverty, tariffs, and other frictions prevent trading and purchasing of various goods, so measuring a single good can cause a large error. The PPP term accounts for this by using a basket of goods, that is, many goods with different quantities. PPP then computes an inflation and exchange rate as the ratio of the price of the basket in one location to the price of the basket in the other location. For example, if a basket consisting of 1 computer, 1 ton of rice, and 1 ton of steel was 1800 US dollars in New York and the same goods cost 10800 HK dollars in Hong Kong, the PPP exchange rate would be 6 HK dollars for every 1 US dollar.

The name purchasing power parity comes from the idea that, with the right exchange rate, consumers in every location will have the same purchasing power.

The value of the PPP exchange rate is very dependent on the basket of goods chosen. In general, goods are chosen that might closely obey the law of one price. So, ones traded easily and are commonly available in both locations. Organizations that compute PPP exchange rates use different baskets of goods and can come up with different values.

The PPP exchange rate may not match the market exchange rate. The market rate is more volatile because it reacts to changes in demand at each location. Also, tariffs and difference in the price of labor (see Balassa–Samuelson theorem) can contribute to longer term differences between the two rates. One use of PPP is to predict longer term exchange rates.

Because PPP exchange rates are more stable and are less affected by tariffs, they are used for many international comparisons, such as comparing countries’ GDPs or other national income statistics. These numbers often come with the label PPP-adjusted.

There can be marked differences between purchasing power adjusted incomes and those converted via market exchange rates.[2] A well-known purchasing power adjustment is the Geary–Khamis dollar (the international dollar). The World Bank’s World Development Indicators 2005 estimated that in 2003, one Geary–Khamis dollar was equivalent to about 1.8 Chinese yuan by purchasing power parity[3]—considerably different from the nominal exchange rate. This discrepancy has large implications; for instance, when converted via the nominal exchange rates GDP per capita in India is about US$1,965[4] while on a PPP basis it is about US$7,197.[5] At the other extreme, for instance Denmark’s nominal GDP per capita is around US$53,242, but its PPP figure is US$46,602, in line with other developed nations.


There are variations to calculating PPP. The EKS method (developed by Ö. Éltető, P. Köves and B. Szulc) uses the geometric mean of the exchange rates computed for individual goods.[6] The EKS-S method (by Éltető, Köves, Szulc, and Sergeev) uses two different baskets, one for each country, and then averages the result. While these methods work for 2 countries, the exchange rates may be inconsistent if applied to 3 countries, so further adjustment may be necessary so that the rate from currency A to B times the rate from B to C equals the rate from A to C.

Relative PPP

Relative PPP is a weaker statement based on the law of one price, covering changes in the exchange rate and inflation rates. It seems to mirror the exchange rate closer than PPP does.[7]



Purchasing power parity exchange rate is used when comparing national production and consumption and other places where the prices of non-traded goods are considered important. (Market exchange rates are used for individual goods that are traded). PPP rates are more stable over time and can be used when that attribute is important.

PPP exchange rates help costing but exclude profits and above all do not consider the different quality of goods among countries. The same product, for instance, can have a different level of quality and even safety in different countries, and may be subject to different taxes and transport costs. 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 measured at PPP controls for the different costs of living and price levels, usually relative to the United States dollar, enabling a more accurate estimate of a nation’s level of production.

The exchange rate reflects transaction values for traded goods between countries in contrast to non-traded goods, that is, goods produced for home-country use. Also, currencies are traded for purposes other than trade in goods and services, e.g., to buy capital assets whose prices vary more than those of physical goods. Also, different interest rates, speculation, hedging or interventions by central banks can influence the foreign-exchange market.

The PPP method is used as an alternative to correct for possible statistical bias. The Penn World Table is a widely cited source of PPP adjustments, and the associated Penn effect reflects such a systematic bias in using exchange rates to outputs among countries.

For example, if the value of the Mexican peso falls by half compared to the US dollar, the Mexican gross domestic product measured in dollars will also halve. However, this exchange rate results from international trade and financial markets. It does not necessarily mean that Mexicans are poorer by a half; if incomes and prices measured in pesos stay the same, they will be no worse off assuming that imported goods are not essential to the quality of life of individuals. Measuring income in different countries using PPP exchange rates helps to avoid this problem, as the metrics gives understanding of relative wealth regarding local goods and services at domestic markets. On the other hand, it is poor for measuring relative cost of goods and services at international markets. The reason is it does not take into account how much 1 USD stands for in a respective country. Using the above mentioned example: at an international market Mexicans can buy less than Americans after the fall of their currency, though their GDP PPP changed a little.

Exchange rate prediction

PPP exchange rates are also valued because market exchange rates tend to move in their general direction, over a period of years. There is some value to knowing in which direction the exchange rate is more likely to shift over the long run.

In neoclassical economic theory, the purchasing power parity theory assumes that the exchange rate between two currencies actually observed in the foreign exchange market is the one that is used in the purchasing power parity comparisons, so that the same amount of goods could actually be purchased in either currency with the same beginning amount of funds. Depending on the particular theory, purchasing power parity is assumed to hold either in the long run or, more strongly, in the short run. Theories that invoke purchasing power parity assume that in some circumstances a fall in either currency’s purchasing power (a rise in its price level) would lead to a proportional decrease in that currency’s valuation on the foreign exchange market.

Identifying manipulation

PPP exchange rates are especially useful when official exchange rates are artificially manipulated by governments. Countries with strong government control of the economy sometimes enforce official exchange rates that make their own currency artificially strong. By contrast, the currency’s black market exchange rate is artificially weak. In such cases, a PPP exchange rate is likely the most realistic basis for economic comparison. Similarly, when exchange rates deviate significantly from their long term equilibrium due to speculative attacks or carry trade, a PPP exchange rate offers a better alternative for comparison.

In 2011, the Big Mac Index was used to identify manipulation of inflation numbers by Argentina. Argentina responded by manipulating the Big Mac Index.[8]


The PPP exchange-rate calculation is controversial because of the difficulties of finding comparable baskets of goods to compare purchasing power across countries.[9]

Estimation of purchasing power parity is complicated by the fact that countries do not simply differ in a uniform price level; rather, the difference in food prices may be greater than the difference in housing prices, while also less than the difference in entertainment prices. People in different countries typically consume different baskets of goods. It is necessary to compare the cost of baskets of goods and services using a price index. This is a difficult task because purchasing patterns and even the goods available to purchase differ across countries.

Thus, it is necessary to make adjustments for differences in the quality of goods and services. Furthermore, the basket of goods representative of one economy will vary from that of another: Americans eat more bread; Chinese more rice. Hence a PPP calculated using the US consumption as a base will differ from that calculated using China as a base. Additional statistical difficulties arise with multilateral comparisons when(as is usually the case) more than two countries are to be compared.

Various ways of averaging bilateral PPPs can provide a more stable multilateral comparison, but at the cost of distorting bilateral ones. These are all general issues of indexing; as with other price indices there is no way to reduce complexity to a single number that is equally satisfying for all purposes. Nevertheless, PPPs are typically robust in the face of the many problems that arise in using market exchange rates to make comparisons.

For example, in 2005 the price of a gallon of gasoline in Saudi Arabia was US$0.91, and in Norway the price was US$6.27.[10] The significant differences in price would not contribute to accuracy in a PPP analysis, despite all of the variables that contribute to the significant differences in price. More comparisons have to be made and used as variables in the overall formulation of the PPP.

When PPP comparisons are to be made over some interval of time, proper account needs to be made of inflationary effects.

In addition to methodological issues presented by the selection of a basket of goods, PPP estimates can also vary based on the statistical capacity of participating countries. The International Comparison Program, which PPP estimates are based on, require the disaggregation of national accounts into production, expenditure or (in some cases) income, and not all participating countries routinely disaggregate their data into such categories.

Some aspects of PPP comparison are theoretically impossible or unclear. For example, there is no basis for comparison between the Ethiopian laborer who lives on teff with the Thai laborer who lives on rice, because teff is not commercially available in Thailand and rice is not in Ethiopia, so the price of rice in Ethiopia or teff in Thailand cannot be determined. As a general rule, the more similar the price structure between countries, the more valid the PPP comparison.

PPP levels will also vary based on the formula used to calculate price matrices. Possible formulas include GEKS-Fisher, Geary-Khamis, IDB, and the superlative method. Each has advantages and disadvantages.

Linking regions presents another methodological difficulty. In the 2005 ICP round, regions were compared by using a list of some 1,000 identical items for which a price could be found for 18 countries, selected so that at least two countries would be in each region. While this was superior to earlier “bridging” methods, which do not fully take into account differing quality between goods, it may serve to overstate the PPP basis of poorer countries, because the price indexing on which PPP is based will assign to poorer countries the greater weight of goods consumed in greater shares in richer countries.

There are a number of reasons that different measures do not perfectly reflect standards of living.

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