I. Assumptions underlying the results of the
2006 revision of World Population Prospects
B. Mortality assumptions: increasing life
expectancy except when affected by HIV/AIDS
C. International
migration assumptions
D.
Eleven Projections Variants
E. Methodological Changes Introduced in the
2006 Revision
II. Definition of
major areas and regions
The preparation of each new revision of the official
population estimates and projections of the United Nations involves two
distinct processes: (a) the incorporation of all new and relevant information
regarding the past demographic dynamics of the population of each country or
area of the world; and (b) the formulation of detailed assumptions about the
future paths of fertility, mortality and international migration. The data
sources used and the methods applied in revising past estimates of demographic
indicators (i.e., those referring to 1950-2005) are presented in volume III of World Population Prospects: The 2006
Revision (forthcoming).
The future population of each country is projected starting with an estimated
population for 1 July 2005. Because population data are not necessarily
available for that date, the 2005 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 2005 using all available data on
fertility, mortality and international migration trends between the reference
date of the population data available and 1 July 2005. In cases where recent
data on the components of population growth 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 necessary1.
To project the population until 2050, the United Nations Population Division
uses assumptions regarding future trends in fertility, mortality and
international migration. Because future trends cannot be known with certainty,
a number of projection variants are produced. The following paragraphs
summarize the main assumptions underlying the derivation of demographic
indicators for the period starting in 2005 and ending in 2050.
The 2006 Revision includes
eight projection variants and three AIDS scenarios. The eight variants are:
low; medium; high; constant-fertility; instant-replacement-fertility;
constant-mortality; no change (constant-fertility and constant-mortality); and
zero-migration. The World Population
Prospects Highlights focuses on the medium variant of the 2006 Revision, and results from the
first four variants are available on-line and are published in volume I of World Population Prospects
(forthcoming). The full set of results for all variants and scenarios are
available only on CD-ROM.
The first five variants, namely, the low, medium, high, constant-fertility and
instant-replacement-fertility, differ among themselves exclusively in the
assumptions made regarding the future path of fertility. The sixth variant,
named “constant-mortality”, differs from the medium variant only with regard to
the path followed by future mortality. The seventh variant, denominated “no
change”, has constant mortality and constant fertility and thus differs from
the medium variant with respect to both fertility and mortality. The eight
variant, denominated “zero-migration”, differs from the medium variant only
with regard to the path followed by future international migration. Generally,
variants differ from each other only over the period 2005-2050.
In addition, the 2006 Revision
includes three AIDS scenarios named No-AIDS, high-AIDS and AIDS-vaccine. These
scenarios are variations of the medium variant and differ from each other and
from the medium variant in terms of the path mortality follows because they
each incorporate different assumptions regarding the course of the HIV/AIDS
epidemic. Note that only 62 countries are considered to be significantly
affected by the epidemic. Consequently, the AIDS scenarios produce different
projections only for those countries.
To describe the different projection variants and scenarios, the various
assumptions made regarding fertility, mortality and international migration are
presented below.
The
fertility assumptions are described in terms of the following groups of
countries:
·
High-fertility countries: Countries that until 2005 had no
fertility reduction or only an incipient decline;
·
Medium-fertility countries: Countries where fertility has been
declining but whose level was still above 2.1 children per woman in 2000-2005;
·
Low-fertility countries: Countries with total fertility at
or below 2.1 children per woman in 2000-2005.
1. Medium-fertility assumption:
Total
fertility in all countries is assumed to converge eventually toward a level of
1.85 children per woman. However, not all countries reach this level during the
projection period, that is, by 2045-2050. Projection procedures differ slightly
depending on whether a country had a total fertility above or below 1.85
children per woman in 2000-2005.
·
Total
fertility in all countries is assumed to converge eventually toward a level of
1.85 children per woman. However, not all countries reach this level during the
projection period, that is, by 2045-2050. Projection procedures differ slightly
depending on whether a country had a total fertility above or below 1.85
children per woman in 2000-2005.
Fertility in high- and medium-fertility countries is assumed to follow
a path derived from models of fertility decline established by the United
Nations Population Division on the basis of the past experience of all
countries with declining fertility during 1950-2000. The models relate the
level of total fertility during a period to the average expected decline in
total fertility during the next period. If the total fertility projected by a
model for a country falls to 1.85 children per woman before 2050, total
fertility is held constant at that level for the remainder of the projection
period (that is, until 2050). Therefore, the level of 1.85 children per woman
represents a floor value below which the total fertility of high- and
medium-fertility countries is not allowed to drop before 2050. However, it is
not necessary for all countries to reach the floor value by 2050. If the model
of fertility change produces a total fertility above 1.85 children per woman
for 2045-2050, that value is used in projecting the population.
In all cases, the projected fertility paths yielded by the models are
checked against recent trends in fertility for each country. When a country’s
recent fertility trends deviate considerably from those consistent with the
models, fertility is projected over an initial period of 5 or 10 years in such
a way that it follows recent experience. The model projection takes over after
that transition period. For instance, in countries where fertility has stalled
or where there is no evidence of fertility decline, fertility is projected to
remain constant for several more years before a declining path sets in.
·
Fertility
in low-fertility countries is generally assumed to remain below 2.1 children
per woman during most of the projection period and reach 1.85 children per
woman by 2045-2050. For countries where total fertility was below 1.85 children
per woman in 2000-2005, it is assumed that over the first 5 or 10 years of the
projection period fertility will follow the recently observed trends in each
country. After that transition period, fertility is assumed to increase
linearly at a rate of 0.05 children per woman per quinquennium.
Thus, countries whose fertility is currently very low need not reach a level of
1.85 children per woman by 2050.
2. High-fertility assumption:
·
Under
the high variant, fertility is projected to remain 0.5 children above the
fertility in the medium variant over most of the projection period. By
2045-2050, fertility in the high variant is therefore half a child higher than
that of the medium variant. That is, countries reaching a total fertility of
1.85 children per woman in the medium variant have a total fertility of 2.35
children per woman in the high variant at the end of the projection period.
3. Low-fertility assumption:
·
Under
the low variant, fertility is projected to remain 0.5 children below the fertility
in the medium variant over most of the projection period. By 2045-2050,
fertility in the low variant is therefore half a child lower than that of the
medium variant. That is, countries reaching a total fertility of 1.85 children
per woman in the medium variant have a total fertility of 1.35 children per
woman in the low variant at the end of the projection period.
4. Constant-fertility assumption:
·
For
each country, fertility remains constant at the level estimated for 2000-2005.
5. Instant-replacement-fertility
assumption:
·
For
each country, fertility is set to the level necessary to ensure a net
reproduction rate of 1 starting in 2005-2010. Fertility varies over the rest of
the projection period in such a way that the net reproduction rate always
remains equal to unity thus ensuring, over the long-run, the replacement of the
population.
1. Normal mortality assumption:
·
Mortality
is projected on the basis of models of change of life expectancy produced by
the United Nations Population Division. These models produce smaller gains the
higher the life expectancy already reached. The selection of a model for each
country is based on recent trends in life expectancy by sex. For countries
highly affected by the HIV/AIDS epidemic, the model incorporating a slow pace
of mortality decline has generally been used to project a certain slowdown in
the reduction of general mortality risks not related to HIV/AIDS.
2. The impact of HIV/AIDS on mortality:
·
In
the 2006 Revision, all
countries with HIV prevalence among persons aged 15 to 49 equal to or greater
than one per cent are considered as seriously affected by the HIV/AIDS epidemic
and their mortality is projected by modelling
explicitly the course of the epidemic and projecting the yearly incidence of
HIV infection. Also considered among the affected countries are those where HIV
prevalence is lower than one per cent but whose population is so large that the
number of individuals infected is large, such as
·
The
model developed by the UNAIDS Reference Group on Estimates, Modelling
and Projections2 is used to fit past estimates of
HIV prevalence provided by UNAIDS for each of the affected countries so as to
derive the parameters determining the past dynamics of the epidemic for each of
them. For most countries, the model is fitted assuming that the relevant
parameters have remained constant in the past. Beginning in 2005, the parameter
PHI, which reflects the rate of recruitment of new individuals into the high-risk
or susceptible group, is projected to decline by half every twenty years. The
parameter R, which represents the force of infection, is projected to decline
by half every thirty years. The reduction in R reflects the assumption that
changes in behaviour among those subject to the risk
of infection, along with increases in access to treatment for those infected,
will reduce the chances of transmitting the virus.
·
In
the 2006 Revision, prevention
of mother-to-child transmission is modelled using
estimated country-specific coverage rates that average 13 per cent in 2005
among the 62 affected countries, but vary between 0 and 90 per cent among them.
These coverage rates are projected to reach 60 per cent, on average, by 2015,
varying between 40 per cent and 100 per cent among the affected countries3. The coverage rate is assumed to
remain constant between 2015 and 2050 at the level reached by 2015 in each of
the affected countries. Among women receiving treatment, the probability of
transmission from mother to child is assumed to be 1 per cent. These
assumptions produce a reduction in the incidence of HIV infection among
children born to HIV-positive women, but the size of the reduction varies from
country to country depending on the level of coverage that treatment reaches in
each of them4.
·
The
survivorship of infected children2 takes account of varying access to paediatric treatment.4
In the 2006 Revision,
HIV-infected children are divided into two groups: (i)
for those infected in-utero, among whom the disease
progresses rapidly, average survival is expected to be 1.3 years, and (ii) for
those infected after birth through breastfeeding, among whom the disease
progresses slowly, average survival is 14 years without treatment.45, Explicit inclusion of paediatric treatment is done via country-specific coverage
rates which average 9 per cent in 2005 but vary between 0 and 99 per cent among
the 62 affected countries. By 2015, the projected coverage is expected to reach
60 per cent, on average, varying from 40 per cent to 100 per cent among the
affected countries.3 Coverage levels remain constant
from 2015 to 2050 at the level reached in each country by 2015. The annual
survival of children receiving treatment is 95 per cent, so that their mean
survival time is 19.5 years and the median survival time is 13.5 years in the
absence of other causes of death.4
·
The
2006 Revision incorporates a
longer survival for persons receiving treatment with highly active
antiretroviral therapy (ART). The proportion of the HIV-positive population
receiving treatment in each country is consistent with estimates prepared by
the World Health Organization, which averaged 25 per cent in 2005 but varied
between 0 and 100 per cent among the 62 affected countries. Coverage is
projected to reach between 40 per cent and 100 per cent by 2015, averaging 60
per cent for the affected countries. Between 2015 and 2050, coverage levels
remain constant at the level reached in each country by 2015. It is assumed
that adults receiving treatment have, on average, a 90 per cent chance of
surviving each year in the absence of other causes of death. Under this
assumption, mean survival time after the initiation of therapy is 9.5 years and
the median survival time is 6.6 years, in the absence of other causes of death.
Therapy is assumed to start at the time full-blown AIDS develops. Without
treatment, infected adults have a mean survival time of two years after the
onset of full-blown AIDS.5
3. No-AIDS assumption:
·
For
each of the 62 countries for which the impact of HIV/AIDS has been taken into
account, mortality is estimated and projected by applying the mortality levels
likely to be exhibited by the non-infected population to the whole population,
thus excluding the direct impacts of the epidemic. Because AIDS started
affecting the populations in the majority of the highly-affected countries
around 1980, the results of the No-AIDS scenario differ from those of the
medium variant not only during the projection period (2005-2050) but also
during part of the estimation period (mainly during 1980-2005). As mentioned
above, in countries highly affected by the HIV/AIDS epidemic, the slow pace of
mortality decline has generally been used to project the reduction of mortality
risks not related to HIV/AIDS.
4. High-AIDS assumption:
·
Mortality
in the high-AIDS scenario is projected by assuming that the parameters of the
model determining the path of the HIV/AIDS epidemic, specifically PHI and R,
remain constant at their 2005 level. This assumption produces in the long run a
relatively high endemic level of the disease. In contrast with the medium
variant which includes the effect of AIDS, the effect of treatment is not
incorporated in the high-AIDS scenario.
5. AIDS-vaccine assumption:
·
The
so-called AIDS-vaccine assumption refers to the ideal case in which a perfectly
effective vaccine against HIV would be instantly available to everyone by 2010.
Under this assumption, mortality is projected by assuming that no new HIV
infections occur as of 2010. In terms of modelling,
this assumption is equivalent to making the force of infection parameter R
become zero in 2010 and remain at that level over the rest of the projection
period.
6. Constant-mortality assumption:
·
Under
this assumption, mortality is maintained constant in each country at the level
estimated for 2000-2005.
1. Normal-migration assumption:
·
Under
the normal migration assumption, the future path of international migration is set
on the basis of past international migration estimates and consideration of the
policy stance of each country with regard to future international migration
flows. Projected levels of net migration are generally kept constant over most
of the projection period.
Zero-migration assumption:
·
Under
this assumption, for each country, international migration is set to zero
starting in 2005-2010.
The 2006 Revision includes eleven
different projection variants or scenarios (table 1). Five of those variants
differ among themselves only with respect to the level of fertility in each,
that is, they share the assumptions made with respect to mortality and
international migration. The five fertility variants are: low, medium, high, constant-fertility
and instant-replacement fertility. A comparison of their results allows an
assessment of the effects that different fertility paths have on other
demographic parameters.
In
addition to the five fertility variants, a constant-mortality variant, a
zero-migration variant and a no change variant (constant-fertility and
constant-mortality) have been prepared. The constant-mortality variant and the
zero-migration variant both have the same fertility assumption (i.e. medium
fertility). Furthermore, the constant-mortality variant has the same
international migration assumption as the medium variant. Consequently, the
results of the constant-mortality variant can be compared with those of the
medium variant to assess the effect that changing mortality has on other
demographic parameters. Similarly, the zero-migration variant differs from the
medium variant only with respect to the underlying assumption regarding
international migration. Therefore, the zero-migration variant allows an
assessment of the effect that non-zero net migration has on other demographic
parameters. Lastly, the no change variant has the same international migration
as the medium variant but differs from the latter by having constant fertility
and constant mortality. When compared to the medium variant, therefore, its
results shed light on the effects that changing fertility and mortality have on
the results obtained.
Lastly,
as part of the modelling of the HIV/AIDS epidemic and
to evaluate its demographic impact, three AIDS mortality scenarios have been
computed. They all share the same fertility and international migration
assumptions as the medium variant. Consequently, the results of the AIDS
mortality scenarios can be compared with those of the medium variant to assess
the impact of HIV/AIDS and the effect of changing mortality, respectively, on
other demographic parameters. The AIDS scenarios are hypothetical and serve
only as a basis for comparison.
Table 1.
Projection variants or scenarios in terms of assumptions for fertility,
mortality and international migration
|
|
Assumptions |
||
|
Projection
variant |
Fertility |
Mortality |
International migration |
|
|
|
|
|
|
Low |
Low |
Normal* |
|
|
Medium |
Medium |
Normal* |
|
|
High |
High |
Normal* |
|
|
Constant-fertility |
Constant as of 2000-2005 |
Normal* |
|
|
Instant-replacement-fertility |
Instant-Replacement |
Normal* |
|
|
Constant-mortality |
Medium |
Constant as of 2000-2005 |
|
|
No-change |
Constant as of 2000-2005 |
Constant as of 2000-2005 |
|
|
Zero-migration |
Medium |
Normal* |
Zero |
|
No-AIDS |
Medium |
No-AIDS since 1980 |
|
|
High-AIDS |
Medium |
High-AIDS as of 2005 |
|
|
AIDS-Vaccine |
Medium |
AIDS-Vaccine as of 2010 |
|
*
Including the impact of HIV/AIDS in 62 countries, as described in section B.2.
The following changes and adjustments were made in the 2006 Revision in relation to
procedures followed in the 2004
Revision.
1 For a general description of the
procedures used in revising estimates of population dynamics, see "Chapter VI. Methodology of the United Nations
population estimates and projections" (pp. 100-104) in World Population Prospects:
The 2004 Revision, Volume III: Analytical Report.
2 "Improved methods and assumptions for estimation of the
HIV/AIDS epidemic and its impact: Recommendations of the UNAIDS Reference Group
on Estimates, Modelling and Projections".
AIDS, vol.
16, pp. W1-W14 (UNAIDS
Reference Group on Estimates, Modelling and
Projections, 2002).
3 UNAIDS, UNICEF, WHO (2007). Children and AIDS - A Stocktaking Report. Actions and
progress during the first year of Unite
for Children, Unite against AIDS (with Statistical
Annexes). See "Table 1. Preventing mother-to-child transmission of
HIV" (pp. 29-31) and "Table 2. Providing paediatric
treatment" (pp. 32-34) - URL: www.unicef.org/uniteforchildren.
4 Stover, J.,
5 Boerma,
J.T., K.A. Stanecki, M.L. Newell, C. Luo, M. Beusenberg, G.P. Garnett,
K. Little, J.G. Calleja, S. Crowley, J.Y. Kim, E. Zaniewski, N. Walker, J. Stover, and P.D. Ghys. 2006. "Monitoring the scale-up of antiretroviral therapy programmes:
methods to estimate coverage." Bulletin of the World Health Organization 84(2):145-150.
6 World Health Organization and
UNAIDS (2006). Progress on Global Access to HIV
Antiretroviral Therapy. A Report
on "3 by 5" and Beyond. March 2006. See "Annex 1.
Estimated number of people receiving antiretroviral therapy, people needing
antiretroviral therapy, percentage coverage and numbers of antiretroviral
therapy sites in low- and middle-income countries" (pp. 71-76).
Go to
II. Definition of major
areas and regions
NOTES:
Countries or areas with a population of less than 100,000 in 2007 are indicated
by an asterisk (*).
The designations employed and the material in this publication do not imply the
expression of any opinion whatsoever on the part of the Secretariat of the
United Nations concerning the legal status of any country, territory or area or
its authorities, or concerning the delimitation of its frontiers or boundaries.
The designation "more developed" and "less developed"
regions are intended for statistical convenience and do not necessarily express
a judgment about the stage reached by a particular country or area in the
development process. The term "country" as used in this publication
also refers, as appropriate, to territories or areas.
More developed regions: They comprise all regions of Europe plus
Northern America, Australia/New Zealand and
Less developed regions: They comprise all regions of Africa, Asia
(excluding
Least developed countries: The group of least developed countries, as
defined by the United Nations General Assembly in 2003, comprises 50 countries,
of which 34 are in Africa, 10 in Asia, 1 in Latin America and the Caribbean,
and 5 in
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SOURCE OF DATA ON POPULATION
In
preparing the 2006 Revision of
the official United Nations population estimates and projections, the
Population Division considered the most recent demographic data available for
each and every country or area of the world. Standard demographic techniques
were used to estimate the population by age and sex for the base year (2005) as
well as trends in total fertility, life expectancy at birth, infant mortality
and international migration up to 2006. The resulting estimates provided the
basis from which the population projections follow.
Surveys
are often the source of the most recent demographic information for developing
countries. Since the 1970s, there have been several multi-national survey
programmes whose results provide key information about fertility or mortality
in a number of countries. The Demographic and Health Surveys Programme (DHS), which started in 1984, and under whose
auspices more than 220 surveys in 75 countries have been carried out in Africa,
Asia and Latin America and the
Another
survey programme has been carried out by the Pan Arab
Project for Child Development (PAPCHILD) of the League of Arab States, working
in collaboration with several international agencies. Its purpose was to gather
information on the determinants of maternal and child health in Arab countries.
The main results of the PAPCHILD surveys are normally included in national
reports published by the countries undertaking such survey. In the present
volume, when results of such surveys were used in preparing the population
estimates and projections of a country, they are identified by the name of the
country, the acronym PAPCHILD and the year to which the survey refers. The Pan
Arab Project for Family Health (PAPFAM) and Gulf Family Health Survey (GFHS)
continue the task initiated by the PAPCHILD programme.
During
the 1990s, UNICEF embarked on a process of helping countries assess progress
for children at end-decade in relation to the World Summit for Children goals, held in 1990 (see UNICEF,
1991). Since then, two rounds of Multiple Indicator Cluster Surveys have been
carried out (MICS and MICS-2) that collected and estimated, inter alia,
information on infant and child mortality, and a third round (MICS-3) is
underway. The mid-decade assessment led to 100 countries collecting data using
the Multiple Indicator Cluster Surveys (MICS), household surveys developed to
obtain specific mid-decade data, or via MICS questionnaire modules carried by
other surveys. By 1996, 60 developing countries had carried out
stand-alone MICS, and another 40 had incorporated some of the MICS modules into
other surveys. In the second phase of their data collection process, the
so-called end-decade assessment, the list of countries participating in the programme was extended. By 2005, MICS-3 has expanded its
scope in its third round to collect 21 of the 48 Millennium Development Goals (MDGs) indicators and to serve as monitoring tool for the
World Fit for Children, as well as for other major international commitments,
such as the UNGASS on HIV/AIDS and the
Finally,
in preparing the 2006 Revision,
demographic information as produced by other United Nations agencies or bodies,
such as: the Economic and Social Commissions for Asia and the Pacific (ESCAP),
for Latin America and the Caribbean (ECLAC/CELADE) and for Western Asia
(ESCWA), as well as the United Nations High Commissioner for Refugees (UNHCR),
the United Nations Children’s Fund (UNICEF) and the World Health
Organization (WHO), was also used or considered. Data from regional
organizations such as the Statistical Office of the European Communities
(EUROSTAT) and the Council of Europe, the Institut
National de la Statistique et des Études
Économiques (INSEE) and the Centre d’Études et de Recherche sur la Population et le Développement
(CERPOD), have also been consulted.
SOURCE OF DATA ON URBAN AND RURAL POPULATION
The
estimates and projections of the urban and rural population are derived from
the estimated and projected percentage urban as published in World Urbanization Prospects: The 2005
Revision (United Nations) applied to the total population estimates and
projections for the medium variant as published in World Population Prospects: The 2006 Revision (United Nations,
forthcoming). Therefore, they differ from the estimates and projections
published in World Urbanization
Prospects: The 2005 Revision, since the latter were obtained using the
total population produced by the 2004
Revision of World Population Prospects.
Estimates
and projections of the urban and rural populations are available only for the
period 1950-2030.
1 The programme
is currently named Measure DHS and information is accessible through their
website at www.measuredhs.com.
2 For more information. see UNICEF's web site at: www.childinfo.org
Go to
I. Assumptions underlying the results of
the 2006 revision of World Population Prospects