POPULATION PROJECTIONS

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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 gt 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

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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

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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 age- and
  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 all-inclusive 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
National population 640054 (P1) 708185 (P2) 773854 (P3) 840603 (P4)
District population 78855 - - -
Ratio of district to ntl pop, 2001 0.1232 (X1) - - -
Projected district pop 87248 (P2 x X1) 95338 (P3 x X1) 103562 (P4 x 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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