The Big Mac index was devised by Pam Woodall of The Economist in 1986, as a light-hearted guide to whether currencies are at their "correct" level. It is based on one of the oldest concepts in international economics, purchasing power parity (PPP), the notion that a dollar, say, should buy the same amount in all countries. In the long run, argue PPP fans, currencies should move towards the exchange rate, which equalizes the prices of an identical basket of goods and services in each country. In this case, the basket is a McDonalds' Big Mac, which is produced in more than 100 countries. The Big Mac PPP is the exchange rate that would leave hamburgers costing the same in the United States as elsewhere. Comparing actual exchange rates with PPP signals whether a currency is undervalued or overvalued. Some studies have found that the Big Mac index is often a better predictor of currency movements than more theoretically rigorous models.