Some people read tea leaves to predict the future. We prefer hamburgers! The Economist began to calculate Big Mac index in 1986. They use it as a light-hearted guide to whether currencies are at their “correct” exchange rate. It is not intended as a precise predictor of exchange rates, but a tool to make economic theory more digestible. The following is the Big Mac Index in Apr. 25th 2002. 详细解释购买力平价说，说明其均衡是如何实现的，为何这是一个长期学说。

正确答案:购买力平价理论三个多世纪的长期发展过程中出现了多种形式，其中最主要的是绝对购买力平价和相对购买力平价两种形式；前者直接与两国的物价水平有关，而后者则与两国的通货膨胀率有关。绝对购买力平价的一般形式是，它说明，汇率取决于由不同货币衡量的可贸易商品的价格水平之比，即取决于不同货币对可贸易商品的购买力之比。相对购买力平价的一般形式是，它意味着汇率的升值与贬值是由两国的通胀率的差异决定的。