Exchange Rate vs Purchasing Power Parity (PPP)
Exchange Rate and Purchasing Power Parity (PPP) are related concepts in AP Economics that students often mix up. An exchange rate is the price of one country's currency expressed in terms of another currency. Purchasing power parity is the idea that exchange rates should adjust so a basket of goods costs the same across countries. Here is how they compare side by side.
It is set in the foreign exchange market by the supply of and demand for currencies. A higher exchange rate (appreciation) makes imports cheaper and exports more expensive. Exchange rates affect net exports and aggregate demand.
PPP is used to compare living standards and real GDP across nations more fairly than market exchange rates, which can be distorted. The 'Big Mac Index' is a popular informal PPP measure.
Exchange Rate vs Purchasing Power Parity: A Market Price Against a Computed Benchmark
| Exchange Rate | Purchasing Power Parity (PPP) | |
|---|---|---|
| Where the number comes from | Trading in the foreign exchange market, set by supply of and demand for the currency | Dividing one country's basket price by the other country's basket price |
| How fast it moves | Continuously, and it can swing several percent in a week | Slowly, because national price levels are sticky |
| What changes it | Interest differentials, expectations, trade flows, speculation, central bank action | Relative inflation only, since it is built from price levels |
| Diagram you draw | Foreign exchange market, with the currency on the horizontal axis | None, it is a computed benchmark rather than a market |
| Converting output with it | Values output at what it could buy internationally, understating places where non traded goods are cheap | Values output at local prices, giving the better living standards comparison |
| Whether it can be wrong | Cannot be, the quoted rate is the price by definition | Often is, and a gap can persist for years before any pull toward it shows up |
Parity predicts an exchange rate, it is not a second kind of exchange rate
Take a fixed basket of goods that costs 50 dollars at home and 300 marks abroad. Dividing one price by the other gives the purchasing power parity rate, 6 marks per dollar. That number is a prediction: if the basket really is identical and freely shippable, arbitrage should push the market rate toward 6. Now suppose the market rate is 8 marks per dollar. Convert 50 dollars at that rate and you get 400 marks, enough to buy one and one third baskets abroad. The basket is cheaper abroad, so the mark is undervalued against parity by 25 percent, since a mark buys 0.125 dollars at the market rate against 0.1667 at parity. Nothing in that calculation changes what you would actually pay to buy marks today. The market rate stays at 8 until currency supply or demand moves it. Parity tells you which direction the pressure runs, not what your bank will charge you this afternoon.
The choice of rate changes measured output by a third in this example
Suppose monthly output per person in the foreign economy is 480 marks. Convert at the market rate of 8 marks per dollar and the figure becomes 60 dollars. Convert at the parity rate of 6 and it becomes 80 dollars, one third higher. Neither number is wrong. The market conversion answers a question about international purchasing power, namely how much foreign output could be turned into dollars and spent abroad. The parity conversion answers a question about living standards, namely how much a person there can actually buy at local prices. Rent, haircuts, bus fares, and restaurant meals never cross a border, and they tend to be cheap where wages are low, so market conversion systematically understates real consumption in lower income countries. When a table reports income per person, check which conversion produced it before drawing a conclusion, because the ranking of two countries can reverse with the choice.
Relative parity is the version worth carrying into an answer
The strict form of parity, that a basket costs the same everywhere once converted, fails almost everywhere it is checked. The weaker form survives better: differences in inflation should show up as changes in the exchange rate. If prices at home rise 3 percent a year and prices abroad rise 9 percent, relative parity predicts the foreign currency loses roughly 6 percent a year against the dollar. That version connects cleanly to the money supply and inflation material you already have. For a question asking what happens to the exchange rate now, use the foreign exchange market instead. Shift demand for the currency or supply of it, then read appreciation or depreciation off the graph. Parity is the long run anchor sitting behind that graph, not a replacement for drawing it.
Frequently asked questions
Why do market exchange rates stay far from purchasing power parity for so long?
Purchasing power parity assumes goods can be shipped and arbitraged, and a large share of any consumption basket cannot be. Rent, medical care, schooling, and haircuts stay where they are produced, so their prices never get equalized. Transport costs, tariffs, and different tax rates block equalization even for physical goods. On top of that, daily currency trading is dominated by financial flows chasing interest rate differences and expectations, and those flows move far faster than national price levels, which adjust over years.
Which rate should you use to compare living standards between two countries?
Parity converted figures give the better living standards comparison, because they price a person's consumption at the goods actually bought locally. Market rate conversion answers a different question, such as how much foreign debt a country can service or how large its economy looks to an investor deciding where to put dollars. Use market rates for anything involving a real cross border payment, and parity rates for anything about how well people live.
Does an undervalued currency have to appreciate?
Undervaluation against parity signals a direction, not a schedule. A currency can sit below its parity value for years while interest rate gaps, capital controls, reserve purchases, or productivity differences hold it there. The Balassa-Samuelson argument explains part of the gap rather than promising it will close: a country with low productivity in traded goods pays low wages, which keeps rent, haircuts, and other non traded services cheap, so its whole price level sits under parity. That piece of the gap narrows only as productivity converges, and it widens again if productivity stalls. Treat parity as a pull, not a forecast with a date attached.
Live Exchange Rates graph. Drag the curves, or open the full version.
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