POPULATION PROJECTIONS

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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 ones 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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