Big Mac indexcurrency is at the theoretically correct exchange rate with another currency. The measure assumes that the theory of purchasing power parity (PPP) holds. The central idea tenet of PPP is that the exchange rates between two currencies should naturally adjust so that the cost of a sample basket of goods should cost the same in one currency. In the Big Mac index the "basket" is simply taken to be a single Big Mac as sold by fast food chain McDonald's. The Big Mac was chosen because it is available in many countries around the world and so allows for the comparison between lots of countries. The Big Mac PPP exchange rate between two countries is obtained by dividing the cost of a Big Mac in one country (in its currency) by the cost of a Big Mac in another country (in its currency). This value is then compared with the actual exchange rate - if it is lower then the first currency is under-valued (according to PPP theory) compared with the second and conversely if it is higher then the first currency is over-valued.
For example, suppose a Big Mac costs ã2 in the United Kingdom and $2.50 in the United States, then the PPP rate is 2/2.50 = 0.8. If in fact the dollar buys 0.55 pounds then the pound is over-valued with the respect to the dollar.
The Big Mac index was introduced by The Economist newspaper in September 1986 and has been published by that paper more or less annually ever since. The index also gave rise to the word Burgernomics.
In January 2004 The Economist introduced a sister tall latte index. The idea is the same except that the Big Mac is replaced by a cup of Starbucks coffee, acknowledging the global spread of that chain in recent years. In a similar vein, in 1997 the newspaper drew up a "coca-cola map" that showed a strong positive correlation between the amount of Coke consumed per head in a country and that country's wealth.
The burger methodology has limitations in its estimates of the PPP. For example local taxes, rates, levels of competition and import duties for burgers may not be representative of the country's economy as a whole. However the Big Mac index has become widely cited by economists.