1. 1
POPULATION PROJECTIONS
Session 3 - Establishing the base
population
Ben Jarabi
Population Studies & Research Institute
University of Nairobi
2. Population Age-sex Structure
Age-sex structure is a map of a population’s
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
3. 3
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
4. 4
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
5. 5
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
6. 6
Evaluation - Digit Preference
Frequently used indices for detecting digit
preference:
Myers
Whipple’s
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
7. 7
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
8. 8
Evaluation - Age Ratios
An age ratio is defined as:
5Px
5ARx = 100
1/2 (5Px-5 + 5Px+5)
where: 5ARx = age ratio for ages x to x+4
5Px = population at ages x to x+4
The larger the departure of this ratio from
100, the larger the error
9. 9
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
10. 10
Evaluation - Sex Ratios
A sex ratio is defined as:
5MPx
5SRx = 100
5FPx
where: 5SRx = sex ratio for ages x to x+4
5MPx & 5FPx = male & female populations,
respectively, at ages x to x+4
The larger the departure of this ratio from 100,
the larger the error
11. 11
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
12. 12
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
13. 13
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
14. 14
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
15. 15
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
16. 16
Smoothing Methods
Methods that preserve the original total:
The Carrier-Farrag and Karup-King-Newton
The Arriaga formula
Arriaga’s “strong smoothing”
Methods that alter the total slightly:
The United Nations method
17. 17
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
19. 19
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)