POPULATION PROJECTIONS Session 3 - Establishing the base population Ben Jarabi Population Studies & Research Institute University of Nairobi 1

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

Types of Errors Content Coverage Age mis-reporting Digit preference Omitting a unit that should have been included Including a unit more than once Including a unit that should not have been included 3 3 3

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 4 4 4

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 5 5 5

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 6 6 6

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 7 7 7

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 8 8 8

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 9 9 9

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 10 10 10

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 11 11 11

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 12 12 12

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 13 13 13

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 14 14 14

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 15 15 15

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 16 16 16

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 17 17 17

Evaluation and adjustment Exercise 18 18 18

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) 19 19 19