Also, with fixed-effects estimators it is not always possible to recover estimates of time-invariant explanatory variables. Data on R&D expenditures comes from the United Nations Statistical Yearbooks (various issues). The data is reported in current values and in local currency units; therefore, I have translated the data into constant U.S. dollars by using the CPI (for each country) and the official exchange rate, both reported by the World Bank Development Indicators (1998a).