The total population of a country may comprise either all usual residents of the country, known as de jure population, or all persons present in the country, known as de facto population, at the time of the census. For purposes of international comparisons, the UN recommends the use of the de facto definition.
Given the highly dynamic nature of underlying phenomena, births, deaths and migrations, population data supplied by the UN (United Nations) are estimates and projections, based on the population census, by which the countries collect and process demographic, economic and social data pertaining to all persons in the country at a specified time.
Official estimates and projections of the UN involve two distinct processes:
The incorporation of all new and relevant information regarding the past dynamics of the population of each country or area of the world.
The formulation of detailed assumptions about the future paths of fertility, mortality and migration.
UN projections offer four variants, known as the low-fertility, medium-fertility, high-fertility and constant-fertility variants, or low, medium, high and constant for short.
Estimates and projections of population are produced by:
UN Population Division: total world population and population by country broken down by sex and age group;
ILO (International Labour Office): working-age population, active population, labour force participation, unemployment;
FAO (Food and Agriculture Organization): agricultural population;
UNESCO (United Nations Educational, Scientific and Cultural Organization): school attending population;
UNHCR (United Nations High Commissioner for Refugees): refugee population.
Source: Common Database: Definitions List, (http://unstats.un.org/unsd/cdb, retrieved Jan 2008.)