POPULATION PROJECTIONS

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POPULATION PROJECTIONS

• Session 3 - Establishing the base population
• Ben Jarabi
• Population Studies Research Institute
• University of Nairobi

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Population Age-sex Structure

• Age-sex structure is a map of a populations
  demographic history
• Although all modern censuses collect information
  on age and sex of a pop., the data often contain
  errors because some people do not know their true
  age while others do not report their age
  accurately

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Types of Errors

• Content
• Age mis-reporting
• Digit preference
• Coverage
• Omitting a unit that should have been included
• Including a unit more than once
• Including a unit that should not have been
  included

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Evaluation - Rationale

• Age structure is very important with respect to
  planning for social and economic necessities in
  human life
• Knowledge of age structure essential to the
  analysis of fertility, mortality migration
• Errors by age sex are replicated repeated in
  pop. projections - hence the need to evaluate and
  adjust where necessary

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Detecting errors age reporting

• Age misreporting may be suggested by
  irregularities evident in indices or graphs
• Population pyramid
• Age and sex ratios
• Cohort comparison
• Summary indices of irregularities in
• age structure or in age-sex structure

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Evaluation - Digit Preference

• Frequently used indices for detecting digit
  preference
• Myers
• Whipples
• Bachi
• Ramachandran
• They provide not only an overall idea of the
  extent of age misreporting but also indicate the
  preference for certain ending age digits

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Evaluation - Age Ratios

• Age ratios for 5-year age groups are used as
  indices for detecting possible age misreporting
• Normally age ratios are expected to be similar
  throughout the age distribution, and all of them
  should be close to a value of 100

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Evaluation - Age Ratios

• An age ratio is defined as
•   5Px
• 5ARx 100
• 1/2 (5Px-5 5Px5)
•  where 5ARx age ratio for ages x to x4 
• 5Px population at ages x to x4
• The larger the departure of this ratio from 100,
  the larger the error

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Evaluation - Sex Ratios

• The level of the sex ratios depends on the number
  of male and female births and on the mortality
  of the population
• All populations have more male than female
  births, and so the sex ratio at the early ages is
  expected to be slightly over 100
• Since mortality is usually higher for males than
  females, the sex ratio is reduced continuously up
  to the oldest ages

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Evaluation - Sex Ratios

• A sex ratio is defined as
•   5MPx
• 5SRx 100
• 
  5FPx
•  where 5SRx sex ratio for ages x to x4 
• 5MPx 5FPx male female populations,
  respectively, at ages x to x4
• The larger the departure of this ratio from 100,
  the larger the error

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The Age-Sex Accuracy Index

• The UN suggested a joint accuracy index to
  summarize the age sex ratios
• The index of sex-ratio score (SRS) is defined as
  The mean difference between sex ratios for the
  successive age groups, averaged irrespective of
  sign
• The index of age-ratio score (ARS) is defined as
  The mean deviation of the age ratios from 100
  percent, also irrespective of sign

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The Age-Sex Accuracy Index

• Based on empirical relationships between the
  sex-ratio scores and the age-ratio scores, the
  following index is defined as the joint score
  (JS) or age-sex accuracy index
• JS 3xSRS ARSM ARSF

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The Age-Sex Accuracy Index

• The age and sex structure of a population will
  be
• accurate if the joint score index is under 20
• inaccurate if the joint score index is between 20
  and 40
• highly inaccurate if the index value is over 40

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Correcting for Age Misreporting

• Smoothing techniques have frequently been used
  for correcting data for age misreporting
• These techniques involve the application of a
  formula to the original data

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Correcting for Age Misreporting

• Smoothing techniques may be classified into 2
  categories
• Those which accept the population in each 10-year
  age group separate it into two 5-year age
  groups without modifying the total population
  size
• Those which smooth the 5-year age groups and
  modify slightly (either up or down) the
  population being smoothed

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Smoothing Methods

• Methods that preserve the original total
• The Carrier-Farrag and Karup-King-Newton
• The Arriaga formula
• Arriagas strong smoothing
• Methods that alter the total slightly
• The United Nations method

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Smoothing Methods

• There is no generalized solution for all
  populations
• The technique to be used will depend on the
  errors in the age and sex distributions
• While, as Arriaga and Associates (1994) note,
  differences in results across procedures are
  small, a decision to use strong smoothing should
  not be taken lightly
• The whole age distribution need not be smoothed
  if only part is considered problematic

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• Evaluation and adjustment
• Exercise

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Inputs - Base Population

• A population by sex age is required to serve as
  the base population for the starting date of the
  projection
• Usually, the base population is taken from the
  latest available census
• Census enumerations are not always perfect - the
  reported data on age and sex may be affected by
  errors hence the need for adjustment
• Where necessary, move the adjusted population
  from a given date (e.g. a census date) to another
  date (e.g. midyear)

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