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Fertility Ideas. (Session 18). Learning Objectives – this session. At the end of this session, you will be able to utilise a variety of commonly-used summaries of fertility data discuss the data sources for, and interpretation of, these measures
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Fertility Ideas (Session 18)
Learning Objectives – this session At the end of this session, you will be able to • utilise a variety of commonly-used summaries of fertility data • discuss the data sources for, and interpretation of, these measures • appreciate some of their limitations in terms of understanding real populations’ approaches to family formation
Introduction Unlike with deaths, there is no probability of 1 in the study of fertility! The demographic section of this module is aiming towards the study of population projections ~ an important area in practice, where LT probabilities have a real role. We introduce fertility as a theme because it too is central in demographic work, and it too is an essential part of making population projections.
Basic measures: CBR, GFR Crude birth rate (CBR) is the number of births per year per 1000 persons in pop.n. Very limited data requirements, but no other benefits. It ignores/conceals effects of e.g. M/F ratios, age composition of the pop.n, age at marriage etc. General fertility rate (GFR) is no. of births per year per 1000 women aged 15-44 in the pop.n [unless an alternative to 15-44 is stated]. Relates births to (effectively all) potential mothers.
Basics: age-specific fertility rates Number of births in 1 year to mothers aged x (or in age group x to < x+n) per 1000 women of that age in the population. Defined in terms of mother’s age at birth of child. Most often, the burden of data collection and presentation by single years of age is reduced and these rates are shown for 5 year age-groups; 15-19, 20-24, 25-29, 30-34, 35-39, 40-44 (6 bands) and in some populations 45-49 (7 bands)
Illustration Artificial data shown opposite for 2 “pop.ns” with similar total childbearing (area under curves). The initially higher one () has children born to women who are on average younger.
Illustration Artificial data shown opposite for 2 “pop.ns” with similar total childbearing (area under curves). The initially higher one () has children born to women who are on average younger. These could represent 2 cohorts in same population, the later-peaking one () having child-bearing deferred e.g. by contraceptive use
Basics: child-woman ratio Where birth registration is not complete, above are not directly available. A census or large, representative sample survey can yield the child-woman ratio i.e. ratio of number of children [surviving] under age 5 to number of women aged 15-44 (or 49). This accounts for some of the under-five mortality, not purely fertility in terms of live births [is this good or bad? It measures something different which we might call “net fertility” (not a standard term!)]
Basics: standardised fertility rates Directly and indirectly standardised birth and fertility rates work in the same way as for death rates, e.g. comparisons of populations with different age-structures are artificially made comparable by applying actual ASFRs to a common age-structure. Issues about choice of a “common” population are same as for death rates.
Generation vs. calendar year Death happens to everyone, it happens once, and generally not at a time of the individual’s choice! The number and spacing of children has more elements of choice as well as many uncertainties, especially if contraception is available, and economic opportunity offers alternatives to early motherhood. It is often desirable to look at the whole of a woman’s child-bearing history.
Example of generation effects In Afristan the average age at first confinement was 20 for women born in 1950, 24 for those born in 1975. Variously attributed to changed • contraceptive availability and/or • employment opportunities for girls and/or • changed social attitudes about family size and responsible parenthood and/or • greater female participation in education (effects ? include increased social awareness, empowerment, biological knowledge, taboo on schoolgirl pregnancies)
Age-period-cohort effects: 1 The accompanying handout Afristan_Generation_fert.xls shows the same set of age-specific fertility rates organised in two ways • rows as “cohorts” by mother’s yr of birth • rows as “current” time ~ the year of childbirth These yield two different generational summary statistics, both making some assumptions about pop.n size & mortality
Age-period-cohort effects: 2 In the handout Afristan_Generation_fert.xls note there is some vagueness in common with genuine data of this type. If someone was “aged 15-19” in “1928-32” she could have been born between 1908 & 1918 i.e. the date attribution of the cohort is indicative, rather than very precise. Note that the figures shown in the handout are average rates per year across each of 5 years.
Average completed family size For a birth cohort of mothers, the average completed family size adds the ASFRs for each of their reproductive years, collected over the relevant 35-year period e.g. 27x5 + 128x5 + … /1000 = 2.43 This is a cohort measure of fertility, and represents the cohort reality, but it is only available after the cohort reaches the end of family formation i.e. after age 50
Total fertility rate For a period e.g. 1975-79 the total (period) fertility rate adds the ASFRs for women of each age e.g. 39x5 + 175x5 + ../1000 = 3.075. This is a form of standardised fertility rate, where the “standard pop.n” has 1000 women at each age. It represents the average number of children that women who survive to age 50 would have if they were subject to the 1975-79 ASFRs throughout their reproductive years.
Gross reproduction rate Based on replacing an adult woman in one generation as a “reproductive engine” by her daughter(s) in the next, the gross reproduction rate is the same as the TFR except it uses only female births, not both sexes. It is frequently expressed per 1 woman rather than per 1000. Taking “”(Greek sigma) to mean “the sum from ages 15 to 49 inclusive”, it can be expressed as GRR = fx where fx is the ASFR for age restricted to female births
Net reproduction rate The GRR assumes daughters live to reproductive maturity, net reproduction rate argues that daughter only “replaces” mother if she survives till age her mother was when she was born. With single-year data, assume mother on average is halfway through her year aged x, so NRR = fx.lx+½ /l0 ≈ fx.Lx/l0. Of course NRR > 1 suggests population increasing, < 1 decreasing … but
Caution: 1 Even though a relatively complex set of ideas goes into a NRR, there are still very many assumptions e.g. that daughter generation will marry/reproduce in same proportion - and at same ages - as their mothers. This would not have worked well in most countries over the last 40 years!
Caution: 2 Ideas above do not incorporate ideas about women’s/couples’ ideas of a desirable completed family size. If they have such ambitions, parity-specific fertility rates and parity progression ratios may contain useful information about the conditional prob. that a woman aged x will have a further child, conditional on already having had m children.
