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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