Purchasing Power Parity
Purchasing Power Parity (PPP) is an economic theory and measurement framework stating that, in the long run, exchange rates between currencies should adjust so that identical goods cost the same in different countries when prices are expressed in a common currency. PPP implies that the equilibrium exchange rate between two currencies equals the ratio of their domestic price levels, and it is used extensively in international economics for comparing GDP across countries, assessing currency misalignment, and making long-run exchange rate forecasts.
Key takeaways
- Absolute PPP states that the exchange rate between two currencies should equal the ratio of their price levels: E(A/B) = P_A / P_B; The Economist's Big Mac Index is the most widely cited informal test of absolute PPP.
- Relative PPP—a weaker and empirically more supported version—states that the percentage change in the exchange rate should equal the inflation rate differential between the two countries over the same period.
- PPP exchange rates are used to convert GDP across countries to make international comparisons: on a PPP basis, China's GDP surpasses the U.S. by this measure because prices for non-traded goods (haircuts, restaurant meals) are much lower in China.
- PPP holds poorly in the short run (exchange rates deviate substantially from PPP for years or even decades) but has some empirical support as a long-run equilibrium concept with half-lives of deviations estimated at 3-5 years.
- The Balassa-Samuelson effect explains why PPP systematically undervalues rich-country currencies: higher productivity in tradeable goods raises wages and service prices across the economy, making overall price levels higher in productive countries without reflecting misalignment.
Explanation
Purchasing Power Parity originated from the 'Law of One Price'—the arbitrage principle that a freely traded identical good should sell for the same price in all markets once exchange rates are accounted for. If a bushel of wheat costs $5 in the U.S. and €4 in Germany, PPP implies the equilibrium EUR/USD exchange rate is $1.25/€. Any deviation creates arbitrage: buy wheat in Germany, ship it to the U.S., and profit from the price differential until prices equalize. In practice, transportation costs, tariffs, taxes, and product differentiation prevent this arbitrage from fully eliminating price differences, but the law of one price provides the theoretical foundation for PPP.
The Big Mac Index, introduced by The Economist magazine in 1986, is the most famous informal test of PPP. The idea is that a McDonald's Big Mac is a reasonably standardized product produced in many countries, allowing cross-country price comparisons. If a Big Mac costs $5.50 in the United States but only $3.00 in China (when converted at market exchange rates), then by the Big Mac index, the Chinese yuan is approximately 45% undervalued relative to the dollar. While the Big Mac Index is imprecise (the Big Mac is not a traded good; the inputs and profit margins differ across countries), it provides an accessible and widely followed informal gauge of currency misalignment.
In the context of GDP comparisons, PPP exchange rates (provided by the World Bank and IMF) are essential for meaningful international comparisons. Market exchange rates are volatile and reflect financial flows as much as real economic activity; they often understate the size of emerging market economies relative to developed ones because services and non-tradeable goods—which constitute roughly 70% of GDP—are cheaper in lower-wage countries. On a PPP-adjusted basis, China's GDP exceeded the U.S. in 2014, making it the world's largest economy, even though in market exchange rate terms the U.S. remains larger. For investors allocating capital across emerging markets, understanding PPP-adjusted valuations is critical for identifying whether a market's equity prices are genuinely cheap or expensive relative to the economic activity they represent.
For currency traders and macro investors, relative PPP—the differential inflation version—is the most actionable variant. Relative PPP predicts that a country with 10% annual inflation should see its currency depreciate by approximately 10% relative to a low-inflation country, to maintain the purchasing power relationship. This relationship holds imperfectly in the short run (because of interest rate differentials, capital flows, and risk appetite) but tends to assert itself over multi-year horizons. A country running 20% annual inflation (Argentina, Turkey) will almost certainly experience significant currency depreciation over a 3-5 year horizon, regardless of short-term capital flow dynamics—a macro investor can structure long-dated currency positions accordingly.
The Balassa-Samuelson effect is a systematic departure from PPP that explains why currencies of rich, productive countries tend to appear overvalued relative to PPP benchmarks. Rapid productivity growth in the traded goods sector of an emerging economy raises wages throughout the economy (including in the non-traded sector), which raises the overall price level relative to trading partners. This makes the currency appear overvalued in PPP terms even though the appreciation reflects genuine real productivity convergence. Japan in the 1970s-1980s and South Korea in the 1980s-1990s both experienced persistent PPP overvaluation by traditional measures as their productivity surged, illustrating that PPP deviations can persist for decades when driven by structural productivity dynamics.
Formula
Absolute PPP: E(d/f) = P_d / P_f; Relative PPP: %ΔE ≈ π_d - π_f (exchange rate change ≈ inflation differential)
Example
An emerging market macro analyst in 2021 constructs a PPP misalignment model for the Turkish lira. Turkish CPI inflation was running at 20% annually versus U.S. CPI at 7%. Relative PPP predicts the lira should depreciate by approximately 13% annually against the dollar to maintain purchasing power parity. However, the Turkish Central Bank, under political pressure, cut interest rates from 19% to 9% despite rising inflation—creating a deeply negative real interest rate and a currency return/inflation differential that made lira assets unattractive. The analyst takes a short lira position. From mid-2021 to end-2022, the lira lost approximately 60% of its value against the dollar—a significantly larger depreciation than PPP alone would have predicted, as both the PPP misalignment correction and a capital flight premium drove the move. The trade generated substantial returns as the analyst's PPP-based framework correctly identified the direction and magnitude of the adjustment.
Related terms
Arbitrage Basis Central Bank Consumer Price Index Convergence Emerging Markets Equity Exchange Exchange Rate Financial Crisis Frontier Markets Inflation