Transcript PPT
POPULATION PROJECTIONS
Session 8 - Projections for sub-
national and sectoral populations
Ben Jarabi
Population Studies & Research Institute
University of Nairobi
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The need for sub-national projections
Sub-national structures need projected
population to quantify likely need and to
help plan for services
National governments need projected pop.
to allocate resources throughout the
country
Ministries need projected population to
implement and monitor programmes
The private sector may be interested in the
growth of specific age groups and in the
growth of households, both of which drive
consumption
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Sub-national projections
Subnational projections - use either a
cohort-component procedure or one of a
number of simpler, less data-demanding
methodologies
Cohort-component projection, which
requires all of the inputs of national
projections plus internal migration by age
and sex
Mathematical or ratio projection, with or
without age-sex detail
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Sub-national projections
Because of the data and computational
requirements involved, we typically see,
and is generally recommended that the
cohort-component projection be applied at
no lower than first subnational
administrative level
Ratio projection suffices at lower than first
subnational administrative levels
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Sub-national projections
Generating sub-national projections that are
both internally consistent and consistent with a
national projection is usually more challenging
than preparing a national projection
Each region presents the same data problems
as the national projection but, in addition,
preserving consistency across regions and
dealing with data problems that are often more
severe than those at the national level adds to
the challenge
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Sub-national projections
A national projection can be generated as
the sum of a series of sub-national
projections, or a national projection may be
prepared first, followed by sub-national
projections with the region with the largest
population serving as residual
For a given country, a hybrid of procedures
may be used. For example, projections for
major regions can be combined into a
national projection and can serve as
separate control totals for provincial or
district projections
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Sub-national projections
Data Requirements
Census age-sex structure and
For cohort-component projection:
Fertility and trend
Mortality and trend
Internal migration and trend
International migration,
or
For mathematical or ratio projection:
Trend in population totals
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Sub-national projections
Begin with base year age-sex structure
Adjust for coverage if possible
Adjust for consistency of under-10
population with fertility and mortality
Smooth at ages 10+ if need be
Ensure that regions add to national totals
by age and sex
Resulting age-sex structure provides initial
population for forcing base-year
consistency between fertility and mortality
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Sub-national projections
Cohort-Component Method - Summary
This method is widely used, relatively easy
to explain, and practical
It permits the use of already available data
and existing theoretical knowledge on the
dynamics of population growth, and it takes
into account causal factors, at least at the
level of basic components and compositional
factors
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Sub-national projections
Cohort-Component Method - Summary
It has the capability to produce consistent &
comparable national & subnational
projections that are easy to update on a
regular basis
Much of the work required to use this
method lies in the in-depth analysis and
development of assumptions for each of the
components of change
However, it also has its shortcomings and
limitations, e.g. it does not explicitly
incorporate socioeconomic determinants of
population change
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Sub-national projections
Ratio Method - Summary
This method suffers from several
shortcomings
They do not account for differences in
demographic composition or for differences
in the components of growth
They provide little or no information on the
projected demographic characteristics of the
population
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Sub-national projections
Ratio Method - Summary
This method suffers from several shortcomings
Because they have no theoretical content, they
cannot be related to theories of population
growth, except perhaps the logistic model, which
is consistent with a Malthusian view of
population dynamics
Consequently, they have limited usefulness for
analyzing the determinants of population growth
or for simulating the effects of changes in
particular variables or assumptions
In addition, they can lead to unrealistic or even
absurd results, even over relatively short
horizons
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Sectoral projections
Projections of households, school enrollment,
poverty, employment, health, and other
population-related characteristics are needed
for many types of planning, budgeting, and
analysis - for simplicity, these are referred to
as socioeconomic projections
Because of the demand for socioeconomic
projections and their close link to projections
of basic demographic characteristics, the
former are often made on the basis of the latter
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Sectoral projections
Projections of socioeconomic
characteristics, however, has two important
features that distinguish them from strictly
demographic projections
One, some socioeconomic characteristics are
directly affected by policy decisions - e.g.
enrollment is usually dictated by entrance
requirements
In such instances, knowledge of public policy
is essential to the production of projections
of socioeconomic characteristics
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Sectoral projections
Two, projections of socioeconomic
characteristics involve achieved
characteristics - those that can change over
one’s lifetime, e.g. marital status, income,
educational attainment, occupation
As a result, projections of socioeconomic
characteristics involve a variety of
assumptions in addition to those for
projections of strictly demographic
characteristics
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Sectoral projections
Two fundamental approaches are
frequently used to prepare
socioeconomic projections
Participation ratio method
Cohort-progression method
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Sectoral projections
Participation ratio method
In this approach, socioeconomic
characteristics are related to demographic
characteristics through the use of ratios
Current and historical data are used to
construct participation ratios – i.e.,
proportions of the population (stratified by
age, sex, and perhaps other demographic
characteristics) that have the socioeconomic
characteristic of interest
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Sectoral projections
Participation ratio method
Once such ratios are established, they can be
projected in a number of ways, e.g. holding
them constant at recent levels, extrapolating
recent trends, or tying them to ratios found in
other areas
The projected ratios are then applied to
population projections for the geographic
area(s) under consideration to obtain a set
of socioeconomic projections
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Sectoral projections
Cohort-progression method
In this approach, projections are developed by
“surviving” people with particular socioeconomic
characteristics
The numbers with the socioeconomic
characteristic or the corresponding participation
ratios are projected on a cohort basis using
information on changes in the numbers or
participation ratios between two previous dates
The conventional form of this method uses ratios
of the number of persons aged a with a particular
socioeconomic characteristic in year t to the
number of persons aged a - y with that
characteristic in year t - y
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Sectoral projections
Cohort-progression method
It is important to remember that cohort
progression ratios represent net cohort
change rather than gross change
This distinction is important because
fundamental patterns may be masked
without knowing the numbers “entering and
exiting” a population
The cohort-progression method in the form
of participation ratios is used less often than
the version of the method that employs
absolute numbers
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