International Economics
Purchasing Power Parity (PPP)
When the same basket should cost the same — and why it usually doesn't
Purchasing power parity is the long-run claim that exchange rates should adjust until the same basket of goods costs the same in any currency. If a Big Mac costs $5.69 in the United States and 25 yuan in China, the implied yuan/dollar rate is 4.39, not the market's 7.2 — the yuan is undervalued by 39 percent on this measure. PPP works in theory, fails in practice over months or years, and reasserts itself slowly over decades. The PPP-adjusted exchange rate is what the IMF, the World Bank, and the Penn World Tables use to compare real GDP across countries — and it is why China's economy is bigger than the United States' at PPP but smaller at market rates.
- Core ideaSame basket → same price after FX
- Foundational paperCassel (1918), revived by Dornbusch (1976)
- Famous indicatorEconomist Big Mac index (1986–)
- Half-life of deviations~3-5 years (Rogoff 1996)
- 2025 PPP rank #1China (~$35T GDP at PPP)
- Reference benchmarksICP rounds, Penn World Tables, World Bank PPP
Interactive visualization
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From the law of one price to PPP
The intellectual root is the law of one price: a single tradeable good, transportable at low cost between two markets, must sell at the same dollar-equivalent price in both, or arbitrageurs would buy in the cheap market and sell in the expensive one until the gap closed.
Aggregate the law of one price across all goods in a representative basket and you get absolute PPP:
S = P_d / P_fwhere S is the spot exchange rate (units of foreign currency per unit of domestic), P_d is the price of the basket in domestic currency, and P_f is the price of the same basket in foreign currency. If the same basket costs $100 in the US and 600 yuan in China, the PPP-consistent rate is 6 yuan per dollar.
The weaker form, relative PPP, drops the level claim and asks only that exchange-rate changes track inflation differentials:
ΔS/S ≈ π_d − π_fRelative PPP is much more defensible empirically — it just says "the country with higher inflation sees its currency depreciate by roughly the inflation gap." Over decades, currencies of high-inflation countries (Argentina, Turkey, Venezuela) systematically depreciate; relative PPP predicts the cumulative move within an order of magnitude.
The Big Mac index, worked end-to-end
The Economist's Big Mac index, launched in 1986 as a "burgernomics" joke that turned out to be remarkably useful, picks a single internationally available product and treats it as the basket. The Big Mac is near-identical across the world — same beef, same bun, same lettuce — so deviations from PPP are mostly about tradeable inputs, local rents, and wages.
Snapshot from a recent Big Mac index round:
- United States: $5.69
- China: 25.00 yuan
- Switzerland: 7.10 francs
- Mexico: 90.00 pesos
- Egypt: 130.00 EGP
The PPP-implied exchange rate for any pair is just the ratio of local prices:
| Pair | PPP rate (Big Mac) | Market rate (approx.) | Local currency status vs USD |
|---|---|---|---|
| USD/CNY | 25.00 / 5.69 = 4.39 | 7.20 | Yuan ~39% undervalued |
| USD/CHF | 7.10 / 5.69 = 1.25 | 0.91 | Franc ~37% overvalued |
| USD/MXN | 90.00 / 5.69 = 15.82 | 20.50 | Peso ~23% undervalued |
| USD/EGP | 130.00 / 5.69 = 22.85 | 49.00 | Pound ~53% undervalued |
The index reliably flags the same currencies as undervalued or overvalued every year — Switzerland and Norway always look expensive, China and India always look cheap. That persistence is itself the puzzle: PPP fails systematically and predictably.
Absolute vs relative PPP
| Absolute PPP | Relative PPP | |
|---|---|---|
| Statement | S = P_d / P_f (level) | ΔS/S = π_d − π_f (change) |
| Underlying assumption | Law of one price for whole basket | Composition of basket roughly stable |
| Holds when | All goods tradeable and frictionless | Inflation differential is dominant FX driver |
| Empirical fit | Almost never at a point in time | Decent over multi-decade horizons |
| Famous failure | Big Mac gaps of 30-50% | 1990s-2000s yen/dollar swings ≫ inflation gap |
| Use case | Cross-country real-income comparisons | Long-run currency forecasts in macro models |
| Speed of mean reversion | Half-life ~3-5 years (Rogoff) | Faster — comparable to inflation pass-through |
Why PPP fails: Balassa-Samuelson
The deepest reason absolute PPP fails systematically is the Balassa-Samuelson effect (Béla Balassa and Paul Samuelson, both 1964). The story:
- Productivity in tradeable sectors (manufacturing, software, agriculture) varies a lot across countries — German workers produce much more per hour than Bangladeshi workers in the same industry.
- By the law of one price, tradeable goods cost roughly the same everywhere. So the higher productivity in rich-country tradeables shows up as higher wages in those sectors.
- Workers in non-tradeable sectors (haircuts, restaurants, hospitals, real estate) demand wages comparable to those in the tradeable sector — labour is mobile within a country.
- But non-tradeable productivity has not risen as much. So firms must charge more to pay those wages. Non-tradeable prices in rich countries are systematically higher.
- The aggregate price level rises with income. PPP-implied exchange rates therefore overstate poor-country currencies' "true" purchasing power if you only look at tradeables.
Empirically, every 1% increase in GDP per capita is associated with roughly a 0.3% real appreciation. This is why PPP-adjusted GDP is systematically higher than market-rate GDP for poor countries — at PPP, India's 2025 GDP is more than three times its market-rate GDP.
Real-world institutions
- International Comparison Program (ICP). Run jointly by the World Bank, IMF, and OECD, the ICP collects 1,000+ goods and service prices from 170+ countries every six years. The 2017 round and the upcoming 2024 round produce the official PPP exchange rates used by international organisations.
- Penn World Tables. Maintained by the University of Groningen, the Penn World Tables (PWT 10.0+) provide PPP-adjusted real GDP, capital, and TFP series for 183 countries from 1950, the standard dataset for comparative growth research.
- Big Mac index. The Economist publishes annually, with data online at economist.com/big-mac-index. Light, flawed, but the most legible PPP teaching tool ever published.
- iPad index, Coca-Cola index. Variants from CommSec and others extend the Big Mac idea to other near-universal goods. The Apple iPad shows much smaller PPP gaps because Apple deliberately prices closer to a global level.
- OECD PPP series. Quarterly PPP estimates for OECD members, used to compute real (PPP-adjusted) wages and prices for cross-country comparisons.
- IMF World Economic Outlook. Reports both market-rate and PPP-rate GDP. China overtook the United States on PPP-rate GDP around 2014 and is roughly 1.7× the US on PPP terms by 2025; on market rates the US remains larger.
Variants and refinements
- Real exchange rate (RER). The PPP-adjusted exchange rate, RER = S × P_f / P_d. Absolute PPP says RER = 1. Empirically, the real exchange rate wanders persistently — but mean-reverts slowly.
- Effective exchange rate. A trade-weighted index of bilateral RERs against major partners. The Fed's Broad Real Effective Exchange Rate is the standard US measure.
- Penn effect. The empirical regularity that price levels rise with income — a direct prediction of Balassa-Samuelson.
- Engel-Rogers (1996) found that the US-Canada border itself accounts for huge price-level gaps, equivalent to thousands of miles of pure distance. This "border effect" is one reason the law of one price fails.
- Sticky-price PPP. Dornbusch (1976) showed that with sticky goods prices and flexible asset prices, exchange rates can overshoot their PPP value in response to monetary shocks, then drift back. Explains short-term excess volatility.
- PPP for index targeting. Some countries (mostly small commodity exporters) use PPP-style price-level rules in monetary frameworks; the New Zealand and Swiss inflation regimes implicitly anchor relative PPP.
Common pitfalls
- Confusing absolute with relative PPP. Absolute PPP is a level claim and almost always fails. Relative PPP is a change claim and roughly works over decades. Newspaper articles that "test PPP" against year-to-year FX moves are using absolute PPP and unsurprisingly find it broken.
- Treating the Big Mac index as a forecast. The index measures present misalignment; it does not predict mean reversion within any specific timeframe. The yuan has been "undervalued" on Big Mac terms for two decades.
- Forgetting non-tradeables. Real estate, haircuts, schools, and government services are 60-70% of consumption in advanced economies. PPP that only tracks tradeables systematically mismeasures real income.
- Mixing PPP-GDP with market-rate GDP. A country's military budget converted at market rates is the cost the central bank actually pays for foreign equipment. Converted at PPP, it measures the resources mobilised domestically. They are different and not interchangeable.
- Assuming PPP holds in the short run. Rogoff (1996) called the PPP puzzle the question of why the half-life of deviations is so long (3-5 years) given how flexible asset prices are. Use PPP for long horizons; use IRP for short horizons.
- Building a portfolio on Big Mac signals. The signal is real but slow. Currencies can stay 30% off PPP for a decade. Carry-trade dynamics often dominate over PPP within investment horizons.
Frequently asked questions
What is purchasing power parity?
Purchasing power parity (PPP) is the theory that, in equilibrium, exchange rates should adjust so that an identical basket of goods costs the same when expressed in a common currency. If a basket costs $100 in the United States and 600 yuan in China, the PPP-implied yuan/dollar rate is 6 yuan per dollar. When the market rate diverges from this — say to 7.2 — the yuan is said to be undervalued relative to PPP. PPP is built on the law of one price for tradeable goods plus arbitrage.
What is the Big Mac index?
The Big Mac index, published by The Economist since 1986, is a light-touch test of PPP using a single, near-identical product sold in roughly 80 countries. If a Big Mac costs $5.69 in the United States and 25.00 yuan in China, the implied PPP exchange rate is 25 / 5.69 = 4.39 yuan per dollar. With the market rate near 7.2, the index says the yuan is undervalued by about 39 percent against the dollar. The Big Mac index is intentionally crude — but it captures the core of PPP in a number anyone can sanity-check.
What is the difference between absolute and relative PPP?
Absolute PPP says the level of the exchange rate equals the ratio of price levels: S = P_d / P_f. It is the strong form, derived from the law of one price applied to a common basket. Relative PPP only says that changes in the exchange rate equal the difference in inflation rates: ΔS/S ≈ π_d − π_f. It is the weaker, more empirically defensible version. Relative PPP holds approximately over multi-decade horizons; absolute PPP rarely holds at any horizon.
Why does PPP fail?
Several reasons. (1) Many goods are not traded — haircuts, restaurant meals, real estate, healthcare. The law of one price only applies to tradeables. (2) Trade barriers — tariffs, quotas, transport costs, regulations — create wedges. (3) Productivity differences in tradeables raise wages economy-wide and pull up non-tradeable prices, the Balassa-Samuelson effect. (4) Markets can take 3-5 years to mean-revert (Rogoff 1996), so deviations of 30-40 percent at any moment are routine.
What is the Balassa-Samuelson effect?
Rich countries have higher productivity in tradeable sectors (manufacturing, software). Wages in those sectors are bid up. Workers in non-tradeable sectors (haircuts, taxis, restaurants) demand similar wages, but their productivity hasn't risen as fast. So non-tradeable prices in rich countries are systematically higher than in poor countries. The price level rises with income, and PPP-adjusted GDP is systematically larger than market-rate GDP for poor countries.
Why does the IMF report GDP at PPP?
Because comparing real living standards across countries with very different price levels requires a common-basket adjustment. At market exchange rates, China's 2025 GDP is about $20 trillion — smaller than the United States. At PPP, China's GDP is over $35 trillion — larger than the United States, because each yuan buys a larger basket of goods in Chinese cities than each dollar buys in American ones. Both numbers are correct; they answer different questions.
Is PPP useful for currency forecasting?
Only at very long horizons. Empirical work (Rogoff 1996; Cheung-Lai 2000) finds the half-life of PPP deviations is 3-5 years for major currencies and longer for many emerging-market pairs. Within a year, exchange rates are dominated by interest-rate differentials, capital flows, and risk sentiment — PPP is overwhelmed. Over 10-30 years, PPP wins: high-inflation currencies depreciate roughly in line with the inflation gap.