can you summarize this Methodology of the United Nations Population Estimates and Projections
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 237 distinct countries or areas comprising the total population of the world. For the 236 countries or areas that had at least 1,000 inhabitants in 2021, the data set contains complete time series of population size by age and sex and of the components of population change—fertility, mortality and international migration—from 1950 until 2100.

The quality of population estimates and projections hinges on the availability of reliable and timely demographic data, including data collected through civil registration and vital statistics (CRVS) systems, population censuses, population registers and household surveys. Recent and historical data on the size of the population and its composition by age and sex, as well as information on levels and patterns of the components of population change, are used for the preparation of population estimates for each country or area.

Recent population counts are critical for obtaining accurate estimates of population size and its composition by age and sex (Spoorenberg, 2020). The principal data source for this purpose is the population census. Most countries conduct a census approximately once per decade. The COVID-19 pandemic disrupted the planning and conduct of population censuses from 2020 onwards and has impaired the functioning of registration and reporting systems in many countries. A recent survey indicated that the pandemic had a negative impact on the preparation, fieldwork or collection phases of the census in some 75 per cent of the 111 countries that responded (United Nations Statistics Division 2021). Some countries had used administrative registers as sources of census data. It is important to assess the impacts of the pandemic on the timely production of complete and reliable population data and to learn from these experiences how to build more efficient and resilient data systems.

A description of the empirical data that inform the latest set of estimates is available under Data Sources. In total, the 2022 revision is based on information from:

1,758 population and housing censuses for 237 countries or areas. In some countries, population registers based on administrative data systems provide the necessary information. Population data from censuses or registers referring to 2015 or later were available for 152 countries or areas, representing 64 per cent of the 237 countries or areas included in this analysis. For 74 countries or areas, the most recent available population count was from the period 2005-2014. For the remaining 11 countries or areas, the most recent available census data were from before 2005.
information on births and deaths from civil registration and vital statistics systems for 169 countries or areas;
2,890 surveys, including demographic and health surveys, conducted in 237 countries or areas, among which 540 were administered in 2010 or later;
official statistics reported to the Demographic Yearbook of the United Nations, and to Eurostat (European Commission);
population registers and other administrative sources on international migration statistics.
In addition to the national data sources described above, the 2022 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*.
A key task in preparing estimates and projections of the world population is to ensure that, for each country, past, current and future trends in fertility, mortality and international migration are consistent with changes in the size of the population and its distribution by age and sex. Various techniques are used to identify the most likely trends in fertility, mortality and international migration. For countries where no or only minimal data are available, demographic and statistical models are used to estimate levels of fertility, mortality and migration. Times series of population estimates, and of the components of population change, are critical inputs for the creation of population projections, as they provide a starting point for the projected future trends.

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. All computations are done by single year of age and by one-year time interval. For demographic balance accounting, population figures are now computed for 1 January (0h) of a given year, and all counts and rates of vital events refer to calendar years (from 1 January until 31 December). 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 2022 revision, the figures from 1950 up to 2021 are treated as estimates, and thus the projections for each country or area begin on 1 January 2022 and extend until 2100. Because population data are not necessarily available for that date, the 2022 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 2022 using all available data on fertility, mortality and international migration trends between the reference date of the population data available and 1 January 2022. 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-scenario 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-scenario projection. In addition, a number of projection scenarios were produced to convey the sensitivity of the medium-scenario projection to changes in the underlying assumptions, and to explore the implications of alternative future scenarios of population change (see Definition of Projection Scenarios).

For further details, see also the report World Population Prospects 2022: Methodology of the United Nations Population Estimates and Projections.

* 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 Global Burden of Disease project (IHME, University of Washington), the International Database (U.S. Census Bureau).