Demographic Analysis and Evaluation Workshop on Demographic Analysis and Evaluation 1

Fertility: Measures & Concepts

Fertility: Introduction To formulate or evaluate policies concerning population growth, information is needed not only on the number of births, but also on trends in the number, proportion, and ages of women having births. Fertility is the third of four components required for population projection. 3

Fertility: Introduction Fertility estimation may use data from a variety of measurement systems, including vital registration, sample surveys, and censuses, and may involve a number of procedures. The methods for measuring fertility, like mortality, depend not only on the quality but also on the detail of the information available. If data are accurate and reliable, fertility can be estimated directly. If not, then specific techniques are required to estimate fertility indirectly. 4

Fertility: Introduction In this part of the workshop we will cover: Sources of fertility data and measures of fertility Direct estimation from registered births, from births reported from a fixed period prior to a census or survey, and from births recorded in a birth history Methods of indirect estimation Comparison of fertility estimates from censuses, surveys, and vital registration systems, and Derivation and use of a composite fertility series We begin with measures of fertility. 5

Fertility: Introduction This discussion follows Chapter 4 of the Census Bureau’s Population Analysis with Microcomputers. The importance of fertility data, techniques for estimating fertility level and pattern, and methods for adjusting fertility data are discussed in more detail in chapter 4. 6

Fertility: Introduction Indices of fertility can be directly calculated by dividing reported births by reported or estimated population. Births are taken from a census, survey, or vital registration system. Population is taken from a census or post-censal estimate. Fertility may also be indirectly estimated. Indirect estimates of fertility provide a check on direct estimates and in some circumstances may be used in place of those direct estimates. 7

Direct Estimation When reliable information on births and population is available from a registration system and census, or from a survey, direct calculations of fertility can be made based on these data. The crude birth rate is one of the most commonly calculated measures of fertility, the easiest to calculate, and one directly related to population growth. However, demographers often prefer to calculate other measures less influenced by age structure. 8

Direct Estimation Age-specific fertility rates, the general fertility rate, age-specific marital fertility rates, and the total fertility rate are used for measuring fertility levels and reproduction. Indices less frequently used or analyzed are the gross reproduction rate and the net reproduction rate; both of these are closely related to the concept of reproduction, or "replacement," of the population. 9

Direct estimates of fertility may be calculated from: Direct Estimation Direct estimates of fertility may be calculated from: Registered births, either total births or births tabulated by age of mother. Births reported by women interviewed during a census or survey in response to a question about children born during a fixed period (typically 12 months) prior to the interview. Births reported by women in response to a series of questions designed to elicit a full or partial birth history. 10

Measures of Fertility Crude birth rate (CBR) is calculated as the number of births occurring in a year divided by the population at midyear, times 1,000. CBR = 1000*B / P For example, the CBR for Hong Kong in 1987 is obtained as follows: 1,000 x 69,811 / 5,613,000 = 12.44 (births) (population) There were 12 births per 1,000 population in Hong Kong in 1987. 11

Measures of Fertility Because fertility, like mortality, varies with age, a comparison between countries based only on the crude birth rate may be misleading. Two populations may have different crude birth rates even if fertility of women at each age is the same in each of them. In another case, women in one population may actually have lower fertility at each age, and the population may still have a higher crude birth rate if there are more women of reproductive age in that population. Again, an age-standardized CBR avoids this problem.

For example, the GFR for Hong Kong in 1986 is obtained as follows: Measures of Fertility General fertility rate. The simplest measure that limits the number of births to women of childbearing age is the general fertility rate, or the number of births in a year per 1,000 women ages 15 to 49. For example, the GFR for Hong Kong in 1986 is obtained as follows: 1,000 x 72,221 / 1,469,300 = 49.2 (births) (women ages 15 to 49)   There were 49 births per 1,000 women of reproductive age in Hong Kong in 1986. 13

Measures of Fertility Although the general fertility rate represents a refinement over the crude birth rate, it still has its limitations. The frequency of births varies by age of women within the span of reproductive ages, and so populations in which women have the same frequency of birth at each age may have different general fertility rates due to differing age structure of women within the reproductive ages. 14

Measures of Fertility Age-specific fertility rates avoid the remaining problem of the age distribution of women affecting the calculated level of fertility by focusing on individual age groups of women. An age-specific fertility rate is calculated as the number of births in a year to mothers of a specific age per woman (or per 1,000 women) of the same age at midyear. 15

Measures of Fertility 16

Measures of Fertility Age-specific fertility rates (ASFRs) follow a fairly standard pattern among women in all populations: rates start from zero at very young ages, rising to a peak sometime in the twenties, then declining gradually until again reaching zero around 50 years of age. Slight variations to the pattern occur, depending on differences in age at marriage, on the proportion of women sexually active (mostly within marital unions), or on the desire and possibility of controlling pregnancies (mostly by using contraception). 17

Age-specific Fertility Rates Share a Common Pattern across Countries 18

Age-specific Fertility Rates Vary across Time 19

Age-specific Fertility Rates Vary across Time 20

Age-specific Fertility Rates Vary across Time 21

Measures of Fertility Although ASFRs properly measure the fertility of women in each age group, it is difficult to use them to make comparisons among populations or within a certain population over time. In addition, they do not easily portray the overall level of fertility. For these reasons, a summary index was developed, known as the total fertility rate. 22

Measures of Fertility The total fertility rate (TFR) represents the average number of children each woman in a group of women would have by the end of her reproductive years if the group had children according to a set of age‑specific fertility rates pertaining to a particular year. In other words, if a group of women have been exposed to a given set of ASFR's from age 15 to age 49, the average number of children they would have by age 50 is the total fertility rate. 23

Measures of Fertility The TFR is derived by cumulating the age‑specific fertility rates (per woman) for all ages of women. When rates are calculated for the seven conventional 5‑year age groups, the TFR is the sum of the rates for each age group, multiplied by five (the width of the age‑group interval). 24

Measures of Fertility 25

Direct Estimation of Fertility ASFRs and TFR may be calculated directly using births for a specific calendar year or other reference period taken from: A registration system A sample registration system A census or survey question about births occurring during a fixed period (usually 12 months) prior to interview A partial or full birth history reported by women interviewed as part of a survey. In direct estimation, women of reproductive age form the denominator for the calculation. The age range most frequently used is 15 to 49. 26

Direct Estimation of Fertility In direct estimation, women of reproductive age form the denominator for the calculation. The age range most frequently used is 15 to 49. 27

Direct Estimation of Fertility Other direct fertility measures: The Gross Reproduction Rate (GRR): Analogous to the Total Fertility Rate, but it refers to female births only. Net Reproduction Rate (NRR): Measures the average number of daughters that will replace each woman in childbearing ages in the next generation, in a population without migration. 28

Indirect Estimation of Fertility Fertility level also may be indirectly estimated. Some of the methods referred to as “indirect techniques” use data gathered for other or general purposes, such as information on age and sex collected in a census… Spreadsheets: RELE REVCBR … or from census or survey data records with some additional information either linking children to their mothers or on relationship of household members to household head. Program: EASWESPOP-FERT 29

Indirect Estimation of Fertility Others use data tabulated from special questions included on census or survey questionnaires, such as information on how many children each woman has had, commonly known as data on "children ever born" per woman. Spreadsheets: PFRATIO REL-GMPZ ARFE-2 ARFE-3 30