POPULATION PROJECTIONS Ben Jarabi Population Studies & Research Institute University of Nairobi

1. POPULATION PROJECTIONS Ben Jarabi Population Studies & Research Institute University of Nairobi

2. Projections - Objectives At the end of the session, participants will be able to: Outline the relevance of pop projections Detect & adjust errors in age reporting Prepare necessary inputs for a pop projection Outline projection methodologies Generate a pop projection at national level Generate a pop projection at sub-national level Generate sectoral pop projections

3. Presentation Outline A. Introduction B. Evaluation and adjustment of census data C. Preparation of inputs for pop projections D. Projection methodologies E. National and provincial projections F. Sub-national population projections G. Sectoral population projections

4. A. Introduction

5. Projections - Definition A population projection is: An extrapolation of historical data into the future An attempt to describe what is likely to happen under certain explicit assumptions about the future as related to the immediate past A set of calculations, which show the future course of fertility, mortality and migration depending on the assumptions used 5

6. Projections - Rationale Primary needs of people in Kenya cannot be gauged rationally without information on expected size, composition and distribution of the population for different geographic units and at various points in time

7. Projections - Rationale Specific areas in which projections are a necessary input: Allocation of resources (planning) Advocacy Research For M&E(basis for target setting; source of denominators esp. non-routine data) To determine drivers of consumption – corporate sector

8. Projections - Inputs • Base population • Assumptions about the course of events (fertility, mortality, migration) • A method by which the assumptions are applied

9. B. Evaluation & Adjustment

10. 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

11. Evaluation - Rationale Age structure is very important with respect to social and economic characteristics - hence the need for accuracy in age and sex structure Knowledge of age structure is essential to the analysis of fertility, mortality, and migration Errors by age & sex are replicated and repeated in population projections

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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 inaccurateif the index value is over 40

21. 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

22. 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

23. 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

24. 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

25. Exercise 1 Evaluation and adjustment

26. C. Preparation of inputs for population projections

27. 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

28. Projections - Inputs Accurate baseline data are critical to producing accurate population projections Population size and age structure Fertility Mortality Net migration

29. Projections - Inputs Base population Population adjusted for coverage, consistency with fertility & mortality, smoothed Projection Fertility and projected fertility Fertility Mortality & projected mortality Mortality Migration & projected migration Migration

30. 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)

31. Projections - Inputs • Current levels of the demographic processes (fertility, mortality, and migration) are determined for the base year and then projected to future years

32. Projecting future fertility Level of fertility has the greatest effect on population growth due to its multiplier effect: additional children born today will have additional children in the future Fertility projections are made by projecting the course of TFR over time, and translating this into ASFRs Normally, long-term projections assume that fertility will eventually stabilize at replacement level, leading to a stationary population

33. Projecting future mortality Mortality projections are based on projecting future life expectancy at birth for males and females Life expectancy (like TFR), is a period measure, and does not reflect the actual experience of a particular individual Mortality projections must also specify how mortality is distributed over different age groups for both sexes

34. Projecting future mortality Future migration levels are more difficult to project than fertility or mortality Although fertility generally has a larger impact on long-term population growth, volatile migrations can exert a strong influence as well In addition, since no single, compelling theory of migration exists, projections are generally based on past trends and current policies; however, data on historical migration are sparse

35. Exercise 2 Preparation of inputs

36. D. Projection methodologies

37. 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) Projections - Methodology

38. Cohort component- projects separately, the components of population change (fertility, mortality & net migration) Ratio method– generates total size of the population but not the age-sex structure With HIV prevalence > 1%, mortality is projected to include HIV/AIDS impact Projections - Methodology

39. Cohort componentmethod This method simulates how a population changes according to its componentsof 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

40. Cohort componentmethod 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

41. Cohort componentmethod 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

42. Cohort componentmethod 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

43. Cohort componentmethod 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 43

44. 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

45. Ratio method The method is based on the ratio of the sub-national unit, say district, population to that of the national population After the ratio of the district to national population is obtained, assumptions are made on the future values of ratios Once the future values of ratios are fixed, the population of the district can be obtained by applying that ratio to the projected national population in that year

46. Ratio method

47. 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

48. 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 48

49. 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 49

50. E. National and provincial projections 50