THIRTY-ONE years ago, The Economist created the Big Mac index as a way of gauging how different currencies stacked up against the dollar. The index is based on the theory of purchasing-power parity, the idea that in the long run, exchange rates should adjust so that the price of an identical basket of tradable goods is the same. Our basket contains one item, a Big Mac.

The latest version of the index shows, for example, that a Big Mac costs $5.30 in America, but just ¥380 ($3.36) in Japan. The Japanese yen is thus, by our meaty logic, 37% undervalued against the dollar.

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