317 Technical Notes 317 These technical notes discuss the sources and methods used to compile the 149 indicators included in this year’s Se- lected World Development Indicators. The notes follow the order in which the indicators appear in the tables. Sources The data published in the Selected World Development In- dicators are taken from World Development Indicators 2000. Where possible, however, revisions reported since the closing date of that edition have been incorporated. In addition, newly released estimates of population and GNP per capita for 1999 are included in table 1. The World Bank draws on a variety of sources for the sta- tistics published in the World Development Indicators. Data on external debt are reported directly to the World Bank by de- veloping member countries through the Debtor Reporting Sys- tem. Other data are drawn mainly from the United Nations and its specialized agencies, from the International Monetary Fund (IMF), and from country reports to the World Bank. Bank staff estimates are also used to improve currentness or consistency. For most countries, national accounts estimates are obtained from member governments through World Bank economic missions. In some instances these are adjusted by staff to ensure conformity with international definitions and con- cepts. Most social data from national sources are drawn from regular administrative files, special surveys, or periodic census inquiries. The Data Sources section following the Technical Notes lists the principal international sources used (as well as the sources cited in the Technical Notes). Data consistency and reliability Considerable effort has been made to standardize the data, but full comparability cannot be assured, and care must be taken in interpreting the indicators. Many factors affect data avail- ability, comparability, and reliability: statistical systems in many developing economies are still weak; statistical methods, coverage, practices, and definitions differ widely; and cross- country and intertemporal comparisons involve complex tech- nical and conceptual problems that cannot be unequivocally resolved. For these reasons, although the data are drawn from the sources thought to be most authoritative, they should be construed only as indicating trends and characterizing major differences among economies rather than offering precise quantitative measures of those differences. Also, national sta- tistical agencies tend to revise their historical data, particularly for recent years. Thus data of different vintages may be pub- lished in different editions of World Bank publications. Read- ers are advised not to compile such data from different editions. Consistent time-series data are available on the World Devel- opment Indicators 2000 CD-ROM. Ratios and growth rates For ease of reference, the tables usually show ratios and rates of growth rather than the simple underlying values. Values in their original form are available on the World Development In- dicators 2000 CD-ROM. Unless otherwise noted, growth rates are computed using the least-squares regression method (see the section on statistical methods below). Because this method takes into account all available observations during a period, the resulting growth rates reflect general trends that are not unduly influenced by exceptional values. To exclude the effects of inflation, constant price economic indicators are used in calculating growth rates. Data in italics are for a year or period other than that specified in the column heading— up to two years before or after for economic indicators, and up to three years for social indicators, which tend to be col- lected less regularly and to change less dramatically over short periods. Constant price series An economy’s growth is measured by the increase in value added produced by the individuals and enterprises operating in that economy. Thus measuring real growth requires esti- mates of GDP and its components valued in constant prices. The World Bank collects constant price national accounts se- ries in national currencies and recorded in the country’s orig- inal base year. To obtain comparable series of constant price data, it rescales GDP and value added by industrial origin to