POPULATION PROJECTIONS Session 8 Projections for sub national

POPULATION PROJECTIONS Session 8 - Projections for sub- national and sectoral populations Ben Jarabi Population Studies & Research Institute University of Nairobi 1

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 2

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 3

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 4

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 5

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 6

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 7

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 forcing base-year consistency between fertility and mortality 8

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 10

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 11

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 12

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 13

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 14

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 15

Sectoral projections § Two, projections of socioeconomic characteristics involve achieved characteristics - those that can change over one’s 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 16

Sectoral projections § Two fundamental approaches are frequently used to prepare socioeconomic projections § Participation ratio method § Cohort-progression method 17

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 18

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 19

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 20

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 21