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Childbearing, or the fertility of human populations, has changed profoundly in the last several centuries. Four aspects are basic for measuring and studying human fertility: age, parity (number of children ever born), length of birth interval, and population reproductivity. Additionally, there are cross-cutting issues of time perspective and of fertility dimensions. The variety of fertility measures at a given time is both a result of the data available and a precondition to expansions in data collection efforts.
Fertility measures are expressed to reflect child-bearing either in the time period in which they occur, or at the end of the (reproductive) lifetime of a cohort. Period fertility rates and analyses are cross-sectional and give a ”snapshot” of a population for a short period of time. A major advantage of period rates is that they are immediately calculable. A second is that they provide the annual contribution to population growth through fertility. Cohort fertility rates and analyses concern a group of persons with a common temporal experience, such as a birth or marriage date. They take into account the events occurring to women (or men) until the end of their reproductive years. More stable than period rates, they provide the means to evaluate long-term population evolution. The main disadvantage in calculating cohort measures is that they require, at minimum, 30 years of data.
Direct measures of fertility are classically obtained from vital registration records, which provide the numerators (births), and from censuses, projections, or continuous registration systems, which provide the population denominators. The crude birth rate (CBR) provides the number of live births per 1,000 population in a given time period. The CBR is a measure of a population’s overall growth, but it can mask — or exaggerate — fertility differences between two populations which have very different age structures. The general fertility rate (GFR) is the number of live births in a time period to women of reproductive age, usually expressed per 1,000 women aged 15 to 44 or 15 to 49. The age-specific fertility rate (ASFR) is the number of births to women of a certain age divided by the number of women in that age group (e.g., women aged 25 to 29). The total fertility rate (TFR) represents the average number of children ever born to a woman if she were to move through her reproductive years maintaining ASFRs of the current time period.
From the 1960s through the 1980s, a plethora of indirect measures was generated which estimated primarily TFRs and secondarily ASFRs in developing countries. This was due to the dual conditions of data deficiency — censuses being then the primary data source — and the nationally and internationally funded family planning and development programs which needed fertility measures to track outcomes. Indirect measures are necessary when vital statistics and large surveys are not available for calculating ASFRs and TFRs — the case when only census data are available. Also, other data are often incomplete, of dubious quality (especially in reference to age), or are based on small sample size; hence indirect measures may provide better estimates than would direct measures. Similarly, indirect measures can aid in data quality evaluation.
Birth interval analysis has not been given as much attention as age-based analysis. But with the growth of large surveys containing many covariates, the study of birth intervals provides information on the dynamics of family growth, control of reproduction, health consequences for mothers and infants, as well as tempo measures for formal demographic analysis.
Particular attention to measures of population replacement, or reproductivity, came into play during the fertility nadirs experienced by Europe and North America between the world wars and in the last decade of the twentieth century. Also, the sustained high fertility rates of many parts of the ”third world” — particularly in the 1970s and 1980s — generated concern about long-run population growth. A set of measures made it possible to map where a country was in terms of replacing itself, and what that portended in the long run.
- Bogue, D., Arriaga, E., & Anderton, A. (eds.) (1993) Readings in Population Research Methodology. United Nations Population Fund, Chicago, IL.
- Siegel, J. S., Swanson, D. A. (eds.) (2004) The Methods and Materials of Demography, 2nd edn. Elsevier Academic Press, San Diego, CA.