Forty years ago, The Economist introduced its now-famous Big Mac Index. It was based on the simple idea that a Big Mac is a damn near perfect universal commodity, and so if you methodically compare its price across countries, it should give you an approximation of the purchasing power parity across a basket of currencies. For example, a Big Mac currently costs US$6.22 in the US (as of July 2026 and according to The Economist). But in Switzerland, it works out to US$9.04, and in Taiwan, it’s U...