THE Big Mac Index is The Economist's light-hearted guide to exchange rates. The index is based on the theory of purchasing-power parity, which says that exchange rates should move to make the price of a basket of goods the same in each country. Our basket contains just one item, a Big Mac hamburger. The exchange rate that leaves a Big Mac costing the same everywhere is our fair-value yardstick. Many of the currencies in the Fed's major-currency index, including the euro, the British pound, Swiss franc and Canadian dollar, are overvalued and trading higher than last year's burger benchmark. Only the Japanese yen could be considered a snip. The dollar still buys a lot of burger in the rest of Asia too. China's currency is among the most undervalued, but a little bit less so than a year ago. The full index is available on our website by 7pm London time on Thursday July 24th.

AP