IT IS usually considered quaint to predict foreign-exchange movements by reference to whether currencies are dear or cheap. Metrics such as The Economist’s Big Mac index, a lighthearted guide to exchange rates, hint at how far currency values are out of whack. But they are often driven further out of kilter by capital flows, by fear and greed, by the interventions of policymakers, and so on.

Since our last look at the index in July, cheap currencies have narrowed the valuation gap against the dollar—almost completely in case of the Canadian dollar (see chart). Fundamentals, such as fair value, seem (at last) to have greater sway in the foreign-exchange market.

The index is based on the idea of purchasing-power parity, which says exchange rates should move towards the level that would make the price of a basket of goods the same in different countries. Our basket contains only one item, but it is found in around 120 countries: a Big Mac hamburger. If the local cost of...Continue reading