The Big Mac index was invented by The Economist in 1986 as a lighthearted guide to whether currencies are at their “correct” level. It is based on the theory of purchasing-power parity (PPP), the notion that in the long run exchange rates should move towards the rate that would equalise the prices of an identical basket of goods and services (in this case, a burger) in any two countries.
Nevertheless, McDonald’s is also using different commercial strategies which can result in huge differences for a product. Overall, the price of a Big Mac will be a reflection of its local production and delivery cost, the cost of advertising (considerable in some areas), and most importantly what the local market will bear — quite different from country to country, and not all a reflection of relative currency values.
In some markets, a high-volume and low-margin approach makes most sense to maximize profit, while in others a higher margin will generate more profit. Thus the relative prices reflect more than currency values.
The Economist sometimes produces variants on the theme. For example, in January 2004, it showed a Tall Latte index with the Big Mac replaced by a cup of Starbucks coffee.