Frank Hollenbeck writes in today’s Mises Daily:
The Economist magazine publishes twice yearly the Big Mac Index (video) which measures the “over- or undervaluation of a currency. Of course, the price of a hamburger has more to do with the cost of labor and rent than with the cost of the bun, meat, or pickles in a Big Mac. It is also a non-traded good, so we should not expect arbitrage to force uniformity in prices across countries or regions. The Economist is trying to sell magazines, so it is justified in being less than precise. However, professional economists should not be using this index, or any index, to identify a currency as undervalued or overvalued. In reality, such statements are total nonsense.
The value of a currency, as reflected in the exchange rate, is determined by supply and demand. The price of rice is not overvalued nor is the price of apples undervalued. Such a statement would be considered idiotic for rice or apples, but is considered justified for exchange rates. Equilibrium exists where the quantity demanded is equal to the quantity supplied. No one is overpaying or underpaying. To suggest that someone is indeed “overpaying” implies the buyer is irrational.