With each successive revision of the World Population Prospects, the Population Division of the United Nations estimates historical demographic trends for the period from 1950 to the present and projects future population trends out to 2100. The estimates are based on all available sources of data on population size and levels of fertility, mortality and international migration for 235 distinct countries or areas comprising the total population of the world.

A description of the empirical data that inform the latest set of estimates is available under Data sources. In total, the 2019 revision is based on information from:

• 1,690 population and housing censuses for 235 countries or areas, including 236 censuses conducted since 2010;
• vital registration of births and deaths from 163 countries or areas;
• 2,700 surveys, including demographic and health surveys, conducted in 235 countries or areas, among which 540 were administered in 2010 or later;
• official statistics reported to the Demographic Yearbook of the United Nations;
• population registers and other administrative sources on international migration statistics.

In addition to the national data sources described above, the 2019 revision has considered international estimates from the following sources:

• refugee statistics from the Office of the United Nations High Commissioner for Refugees (UNHCR);
• estimated time series of adult HIV prevalence and coverage of antiretroviral treatment from the Joint United Nations Programme on HIV/AIDS (UNAIDS);
• estimated time series of infant and under-five mortality from the United Nations Inter-agency Group for Child Mortality Estimation (UN-IGME);
• estimates of international migration flows and stocks of foreign-born persons from the United Nations;
• various other series of international estimates produced by international and regional organizations, and academic research institutions*.

These data sources served to reconstruct population changes in each country or area from 1950 until the present. In doing so, the Population Division used the cohort-component method (United Nations, 1956) to ensure internal consistency by age and sex and over time, and between the three demographic components of change (fertility, mortality and migration) and the enumerated population. The cohort-component method was also used to project population trends until 2100 using a variety of demographic assumptions concerning the components of population change.

In the 2019 revision, the figures from 1950 through the period from mid-2015 to mid-2020 are treated as estimates, and thus the projections for each country or area begin on 1 July 2020 and extend until 2100. Because population data are not necessarily available for that date, the 2020 estimate is derived from the most recent population data available for each country, obtained usually from a population census or a population register, projected to 2020 using all available data on fertility, mortality and international migration trends between the reference date of the population data available and 1 July 2020. In cases where data on the components of population change relative to the past 5 or 10 years are not available, estimated demographic trends are projections based on the most recent available data. Population data from all sources are evaluated for completeness, accuracy and consistency, and adjusted as necessary.

In projecting future levels of fertility and mortality, probabilistic methods were used to reflect the uncertainty of the projections based on the historical variability of changes in each variable. The method takes into account the past experience of each country, while also reflecting uncertainty about future changes based on the past experience of other countries under similar conditions. The medium-variant projection corresponds to the median of several thousand distinct trajectories of each demographic component derived using the probabilistic model of the variability in changes over time. Prediction intervals reflect the spread in the distribution of outcomes across the projected trajectories and thus provide an assessment of the uncertainty inherent in the medium-variant projection. In addition, a number of projection variants were produced to convey the sensitivity of the medium-variant projection to changes in the underlying assumptions, and to explore the implications of alternative future scenarios of population change (see Definition of projection variants).

For further details, see also the report World Population Prospects 2019: Methodology of the United Nations Population Estimates and Projections.
La versión en español también está disponible aquí:
https://www.cepal.org/es/publicaciones/45989-perspectivas-la-poblacion-mundial-2019-metodologia-naciones-unidas-estimaciones

* Including the Human Mortality Database and Human Life Table Database (UC Berkeley, MPIDR and INED), the Human Fertility Database and Human Fertility Collection (MPIDR and VID), the Latin American Mortality Database–LAMBdA (University of Wisconsin-Madison), the International Data Base (U.S. Census Bureau), the Global Burden of Disease project (IHME, University of Washington) and the Developing Countries Mortality Database–DCMD (Zhejiang University).