Below are the methodologies used by the World Bank to compile statistical data. They cover information from indicator definitions to aggregation rules and other World Bank processes in collecting data:

Data Consistency

Differences in timing and reporting practices may cause inconsistencies among data from different sources, so users should be cautious when combining these data.

Our comprehensive publications World Development Indicators and International Debt Statistics contain data that generally rely on official sources, although some adjustments are made in the balance of payments to account for fiscal/calendar-year differences. Within these publications we attempt to present data that are consistent in definition, timing and methods. Even so, updates and revisions over time may introduce discrepancies from one edition to the next.

National accounts and balance of payment data come from two sources: current reports gathered by the Bank's country management units and data obtained from official sources.

Aggregation Rules

Aggregates are based on the World Bank's regional and income classification of economies. Because of missing data, aggregates for groups of economies should be treated as approximations of unknown totals or average values. Regional and income group aggregates are based on the largest available set of data. The aggregation rules are intended to yield estimates for a consistent set of economies from one period to the next and for all indicators. Small differences between sums of subgroup aggregates and overall totals and averages may occur because of the approximations used. In addition, compilation errors and data reporting practices may cause discrepancies in theoretically identical aggregates such as world exports and world imports.

Growth Rates

Growth rates are calculated as annual averages and represented as percentages. Except where noted, growth rates of values are computed from constant price series. Three principal methods are used to calculate growth rates: least squares, exponential endpoint, and geometric endpoint. Rates of change from one period to the next are calculated as proportional changes from the earlier period.

World Bank Atlas Method

In calculating gross national income (GNI -- formerly referred to as GNP) and GNI per capita in U.S. dollars for certain operational purposes, the World Bank uses the Atlas conversion factor. The purpose of the Atlas conversion factor is to reduce the impact of exchange rate fluctuations in the cross-country comparison of national incomes.

The Atlas conversion factor for any year is the average of a country's exchange rate (or alternative conversion factor) for that year and its exchange rates for the two preceding years, adjusted for the difference between the rate of inflation in the country, and through 2000, that in the G-5 countries (France, Germany, Japan, the United Kingdom and the United States). For 2001 onwards, these countries include the Euro Zone, Japan, the United Kingdom and the United States. A country's inflation rate is measured by the change in its GDP deflator.

Alternative Conversion Factors

The World Bank systematically assesses the appropriateness of official exchange rates as conversion factors. An alternative conversion factor is used when the official exchange rate is judged to diverge by an exceptionally large margin from the rate effectively applied to domestic transactions of foreign currencies and traded products. This applies to only a small number of countries. Alternative conversion factors are used in the Atlas methodology and elsewhere in the World Development Indicators as single-year conversion factors.