Frequently Asked Questions

1. What is the most recent Revision of World Population Prospects?

2. When will the next Revision of the World Population Prospects be released?

3. What is the estimation period for the 2012 Revision of the World Population Prospects?

4. What is the projection period for the 2012 Revision of the World Population Prospects?

5. Why are successive Revisions of the World Population Prospects issued every two years?

6. What country aggregates are available for the World Population Prospects?

7. Where can I find population estimates for periods before 1950?

8. Why does the Population Division produce estimates instead of using official national data?

9. Why do estimates in the World Population Prospects sometimes differ from official statistics?

10. Why is only one variant of future mortality used in producing the projections?

11. Who is using the results of the World Population Prospects?

12. Why will volumes 1 and 2 of the World Population Prospects no longer be available in print?

13. Which data sources have been used for estimating fertility in the World Population Prospects?

1. What is the most recent Revision of World Population Prospects?

The 2012 Revision is the most recent revision of World Population Prospects, the official United Nations population estimates and projections. It was released on 13 June 2013.

2. When will the next Revision of the World Population Prospects be released?

The Population Division of the Department of Economic and Social Affairs of the United Nations issues a new Revision every two years. The next one is due in the first half of 2015.

3. What is the estimation period for the 2012 Revision of the World Population Prospects?

The 2012 Revision provides population estimates for the period 1950-2010. Estimates of stocks are presented for every year that is a multiple of five from 1950 to 2100. Estimates of the components of population change are presented for five-year periods, starting with 1950-1955 and ending with 2095-2100. Stock estimates refer to 1 July of the year in question. Period estimates may be assumed to refer to the mid-point of the period concerned (e.g. the mid-point of the period 1 July 1970 to 1 July 1975 is the 1 January 1973). Data presented by single calendar years or single groups of age are derived by interpolation.

4. What is the projection period for the 2012 Revision of the World Population Prospects?

The 2012 Revision provides population projections for the period 2010-2100. There is only one series for the population estimates, but there are several series for the population projections, because different projection variants are calculated. Calculations are carried out by five-year periods using data classified by five-year age group. 

5. Why are successive Revisions of World Population Prospects issued every two years?

Governments have instructed the Population Division to follow a biennial cycle for the preparation of population estimates and projections. All Revisions of World Population Prospects provide a historical time series of population indicators starting in 1950. The 2012 Revision provides projections up to 2100. Every new Revision takes into account newly released national data to revise estimates of past trends in fertility, mortality or international migration and to update population age distributions on the basis of newly available census data. For more information on the data used in preparing the 2012 Revision, please refer to Data Sources.

6. What country aggregates are available?

The 2012 Revision of World Population Prospects presents aggregated data and indicators for 4 development groups, 6 major areas and 21 geographical regions. In addition, data for special country groups are available on demand, including for sub-Saharan Africa, the land-locked developing countries and the income groups as defined by the World Bank income groups. These full set of special groups will be included among the data sets available for purchase on CD-ROM (to be released tentatively by Sept 2013).

7. Where can I find population estimates for periods before 1950?

For a series of world population estimates starting in year zero, please refer to Table 1: World Population From Year 0 to Stabilization in The World at Six Billion, United Nations, 1999, p. 5.

For a review of world population estimates prior to 1950 prepared by the United Nations see: John C. Caldwell and Thomas Schindlmayr (2002). Historical Population Estimates: Unraveling the Consensus, Population and Development Review, Vol. 28, No. 2 (Jun., 2002), pp. 183-204.

8. Why does the Population Division produce estimates of past population trends instead of using official national data?

World Population Prospects presents estimates for 233 countries and areas. About half of those countries or areas do not report official demographic statistics with the detail necessary for the preparation of cohort-component population projections. The Population Division undertakes its estimation work in order to close those gaps. The availability of data gathered by major survey programs, such as the Demographic and Health Surveys or the Multiple-Indicator Cluster Surveys, are useful in generating some of the data that is not currently being produced by official statistics.

9. Why do the estimates in World Population Prospects sometimes differ from official statistics?

Official statistics are not very perfect. All data have deficiencies. Official demographic statistics are affected by incompleteness of coverage, lack of timeliness and errors in the reporting or coding of the basic information. The analysis carried out by the Population Division takes into account those deficiencies and seeks to establish past population trends by resolving the inconsistencies affecting the basic data. Use of the cohort-component method to reconstruct populations is the major tool to ensure that the population trends estimated by the Population Division are internally consistent.

10. Why is only one variant of future mortality used in producing the United Nations projections?

Over the medium-term future, changes in fertility are more likely to have sizable impacts on future population size, growth and age structure than changes in mortality. Furthermore, whereas there is universal agreement that reducing mortality is a worthy goal, there are varied perspectives on what are the fertility trends best suited to satisfy the goals of different societies. Hence, there is more demand for exploring the effects of different trends in future fertility than there is for exploring those of possible future variations in mortality.

11. Who is using the results of World Population Prospects?

The users of World Population Prospects are many and varied. All entities of the United Nations system use the Population Division's population estimates for the calculation of indicators that require population as an input. The Division's population projections are also used in projecting other population-related variables or in modeling complex systems that use population is an exogenous variable. In addition, several entities and organizations of the United Nations distribute the results of World Population Prospects through their own databases and websites (e.g., the World Bank, the Statistics Division/DESA, the Food and Agriculture Organization). In addition, the data are used by many groups belonging to civil society, from school-children learning about population to journalists to academics. Being the official United Nations population estimates and projections, the results of World Population Prospects are considered to embody the authoritative view of population levels, trends and characteristics.

12. Why will volumes 1 and 2 of World Population Prospects no longer be available in print?

In an effort to "Go Green" and reduce costs, the United Nations Population Division will issue the two volumes summarizing the results of the 2012 Revision of World Population Prospects only in pdf format so that they can be downloaded via the internet. Volume 1 will contain the set of Comprehensive Tables showing the values of each demographic indicator over time and for all major areas, regions and countries. Volume 2 will contain Country Profiles displaying time series of relevant demographic indicators for each of the major areas, regional groups and countries at a time. A volume containing printouts of the sex and age distributions of populations will no longer be produced. The data by age and sex will be available only on Excel spreadsheets or through the interactive database accessible via the internet. The two volumes are expected to be released by the third quarter of 2013.

13. Which data sources are used for estimating fertility?

The fertility estimates for the World Population Prospects are based on the following sources:

Vital Registers
Most developed and some developing countries have vital registration systems, which record the number of births and the age of the mother. This information can be combined with estimates of the female population by age to calculate age-specific fertility rates and total fertility. Countries either report the (age-specific) rates or the number of births by age of mother.

Surveys
Most developing countries do not have vital registration systems, or they have only vital registration in urban areas. In those countries surveys are typically used to estimate fertility. The surveys are carried out by various institutions, from National Statistical Offices to international organizations. The most widely used surveys are the Demographic and Health Surveys (DHS) and the Multi-Indicator Cluster Surveys (MICS). In some countries, there are also Demographic Surveillance Sites (DSS), which provide in-depth fertility and health information for particular areas within these countries. Data from surveys are typically available in the form of birth histories (either full histories or histories truncated for the last 5 years) or as the number of "children ever born". Various indirect methods can be used to analyze and adjust fertility estimates from these surveys - particularly for historical time periods.

Censuses
The third major source of fertility information are the censuses, which typically provide information on the number of children ever born, or on the number of children born during the past 12 or 24 months for each woman age 15 to 49 enumerated. Census data, which provide full records for everyone living in a particular household, also allow application of various indirect measures to estimate fertility, such as the "own children method", which is based on the children of the women living in the household.

Adjustments
Fertility information from all data sources, including censuses, may require adjustments - particularly for historical time series. There are also various demographic techniques to adjust under-reporting in particular age-groups. One particular method attempts to reconstruct the fertility of cohorts across different censuses. For details of the methods used in the World Population Prospects see the section on Data Sources.