In the Economy category, GDP dollar estimates for countries are reported both on an official exchange rate (OER) and a purchasing power parity (PPP) basis. Both measures contain information that is useful to the reader. The PPP method involves the use of standardized international dollar price weights, which are applied to the quantities of final goods and services produced in a given economy. The data derived from the PPP method probably provide the best available starting point for comparisons of economic strength and well-being between countries. In contrast, the currency exchange rate method involves a variety of international and domestic financial forces that may not capture the value of domestic output. Whereas PPP estimates for OECD countries are quite reliable, PPP estimates for developing countries are often rough approximations. In developing countries with weak currencies, the exchange rate estimate of GDP in dollars is typically one-fourth to one-half the PPP estimate. Most of the GDP estimates for developing countries are based on extrapolation of PPP numbers published by the UN International Comparison Program (UNICP) and by Professors Robert Summers and Alan Heston of the University of Pennsylvania and their colleagues. GDP derived using the OER method should be used for the purpose of calculating the share of items such as exports, imports, military expenditures, external debt, or the current account balance, because the dollar values presented in the Factbook for these items have been converted at official exchange rates, not at PPP. One should use the OER GDP figure to calculate the proportion of, say, Chinese defense expenditures in GDP, because that share will be the same as one calculated in local currency units. Comparison of OER GDP with PPP GDP may also indicate whether a currency is over- or under-valued. If OER GDP is smaller than PPP GDP, the official exchange rate may be undervalued, and vice versa. However, there is no strong historical evidence that market exchange rates move in the direction implied by the PPP rate, at least not in the short- or medium-term. Note: the numbers for GDP and other economic data should not be chained together from successive volumes of the Factbook because of changes in the US dollar measuring rod, revisions of data by statistical agencies, use of new or different sources of information, and changes in national statistical methods and practices.