This paper surveys the measurement of trade costs: what we know and don't know but may usefully attempt to learn. Partial and incomplete data on direct measures of costs go with inference on implicit costs from trade flows and prices. Total trade costs in rich countries are large. The ad valorem tax equivalent is about 170 percent when pushing the data hard. Poor countries face even higher trade costs. There is a lot of variation across countries and across goods within countries, much of which makes economic sense. In our survey, theory provides interpretation and perspective and suggests improvements for the future. Some new results are presented to properly apply and interpret gravity theory and handle aggregation.

While empirical evidence finds only a weak relationship between nominal exchange rates and macroeconomic fundamentals, forex markets participants often attribute exchange rate movements to a macroeconomic variable. The variables that matter, however, appear to change over time and some variable is typically taken as a scapegoat. For example, the current dollar weakness appears to be caused almost exclusively by the large current account deficit, while its previous strength was explained mainly by growth differentials. In this paper, we propose an explanation of this phenomenon in a simple monetary model of the exchange rate with noisy rational expectations, where investors have heterogeneous information on some structural parameter of the economy. In this context, there may be rational confusion about the true source of exchange rate fluctuations, so that if an unobservable variable affects the exchange rate, investors may attribute this movement to some current macroeconomic fundamental. We show that this effect applies only to variables with large imbalances. The model thus implies that the impact of macroeconomic variables on the exchange rate changes over time.