1. POPULATION PROJECTIONS
Session 6 - Introduction to population projections
Ben Jarabi
Population Studies & Research Institute
University of Nairobi 1

2. 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) 2 2 2

3. 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 3 3 3

4. 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

5. 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

6. 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 6 6 6

7. 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 7 7 7

8. 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 8 8 8

9. 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 9 9 9

10. 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 10 10 10

11. 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 11 11 11

12. 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 12 12 12

13. 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 13 13 13

14. Ratio method Year 2001 2006 2011 2016 National population 640054
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) 14 14 14

15. 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 15 15 15

16. 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 16

17. 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 17