Transcript PPT

POPULATION PROJECTIONS
Session 6 - Introduction to
population projections
Ben Jarabi
Population Studies & Research Institute
University of Nairobi
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Projections - Methodology

Projection methodologies can be divided
into two main categories:

procedures for projecting the population
considering fertility, mortality, and
migration, by age and sex (component
method)

procedures for projecting the population
using mathematical functions applied to
population figures but not to each of the
components (ratio method)
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Projections - Methodology
 Cohort component - projects separately, the
components of population change (fertility,
mortality & net migration)
 Where HIV prevalence > 1%, mortality should be
projected to include impact of HIV/AIDS impact
 Ratio method – adjusts a population
distribution to an assigned total in proportion
to the frequencies in this distribution
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Projections - Methodology
 Although many of the factors affecting the
methodology and analysis of population
projections are the same for all geographic
areas, there are important differences as well
 Data are more readily available & more reliable
at national than at sub-national level
 Migration typically plays a greater role in pop.
growth at sub-national than at national level
 Population growth rates are generally more
variable at sub-national than at national level
 Hence, choices regarding data, techniques,
and assumptions may be different for
projections at one geographic level than for
projections at another
Projections - Methodology
 In the cohort-component method, the main
difference between national and sub-national
projections is the addition of the component
on internal migration
 Although an assumption that future
international migration will be negligible can
be justified for many countries, internal
migration plays a significant role in almost
every country, and at the sub-national level, it
is often the most important and complex
component of population change
Cohort component method
 This method simulates how a population
changes according to its components of
growth: fertility, mortality, and migration
 Based on past information, assumptions are
made about future trends in these
components of change
 Then, the projected rates are applied to the
age and sex structure of the population, in
a simulation taking into account that people
die according to their sex and age, that
women have children, and that some
people change their residence
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Cohort component method
 Base population is grouped into cohorts
defined by age and sex
 The projection proceeds by updating the
population of each age- and sex-specific
group according to assumptions about
three components of population change
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Cohort component method
 Each cohort survives forward to the next
age group according to assumed ASMRs
 Migration is accounted for by applying ageand sex-specific net migration rates to each
cohort as well
 Projected ASFRs rates are applied to the
female population in childbearing ages to
estimate the number of births
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Cohort component method
 A sex ratio at birth is used to divide total
births into males and females
 These births are exposed to the
appropriate mortality schedule and then
the survivors fed into the projection
model
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Cohort component method
 Time span
 No standard time span over which a
projection should be made
 Select a span that is equal to the maximum
length of time required for completion of the
planned activities
 NB: the longer the time span, the greater the
potential deviation of the projected from the
actual population
 It is usually most convenient to project
population by time intervals equal to the
age intervals
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Ratio method
 Ratio method is applied mainly for
projecting the population of small areas
within a country for which all inputs
required by the component method are not
always readily available
 The method is also useful in the projection
of urban and rural populations
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Ratio method
 This method is used where an area containing
the population to be projected (say district) is
part of a larger (“parent”) area for which
projections are available
 The small areas should exist in a perfect
hierarchical structure - where geographic units at
each level are mutually exclusive & exhaustive and can
be aggregated to higher levels, culminating in one allinclusive unit
 The main drawback of this method is that it
assumes that all the smaller areas will grow at
the same rate as the parent area
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Ratio method
 After the ratio of the district to national
population is obtained, assumptions are
made on the future values of these
ratios
 Once the future values of ratios are
fixed, the population of the district can
be obtained by applying those ratios to
the projected national population in
respective years
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Ratio method
Year
2001
2006
2011
2016
640054
708185
773854
840603
population
(P1)
(P2)
(P3)
(P4)
District
population
78855
-
-
-
Ratio of
district to
ntl pop,
2001
0.1232
-
-
-
87248
95338
103562
(P2 x X1)
(P3 x X1)
(P4 x X1)
National
Projected
district
pop
(X1)
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Ratio method
 Once the projection for each small area has
been made, ensure the sum of the
population of all small areas tallies with the
national total
 Using the national total as a control, adjust
proportionally the projections of the small
areas
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Population projection
 There is no single method or technique that
can improve accuracy
 Accuracy depends on the quality of the input
data and the assumptions made about the
course of future change
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Population projection
 Identifying which projection method is optimal
for a specific type of projection depends on
several factors
 Of crucial importance is whether the projections
are to be carried out for larger geographical
areas (e.g. nations and groups of countries)
where uncertainty is lower, or smaller areas
(e.g. sub-national, urban) where migration
makes future population changes more volatile
and projections more difficult
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