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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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