Age-specific fertility rate

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This article describes a ratio measure. In the standard operationalization, the numerator of the measure is number of live births to females of a particular age (view other ratio measures with this numerator) and the denominator of the measure is number of females of a particular age (view other ratio measures with this denominator). The denominator used for reporting is 1000. The measurement period is a year.
The numerator counts the number of occurrences of an event during the measurement period.
The denominator counts the total count of some type of entity, and an appropriate convention is set regarding the time within the measurement period that this count is calculated.

Definition

Conceptual definition

The age-specific fertility rate (ASFR) refers to the number of live births that females of a given age have on average in a year.

Operationalization

The age-specific fertility rate for an age x may be defined in two related ways that yield slightly different numbers (mathematically) but not by too much.

  • Non-preferred operationalization:It could be defined as the ratio of the number of live births in a given year to females who, at the time of giving birth, had age x, to the size of the population of all females who had their age x birthday at some time in the year (typically, the midpoint).
  • Preferred operationalization: We could fix a birth cohort of females whose birthdate for age x falls somewhere within the year. We could then consider the ratio of the number of recorded live births in the given year to that birth cohort, to the total population of that birth cohort.

Note that the definitions differ subtly in that the numerator for the former is interested in the age of the female at the time of giving birth, whereas the numerator for the latter is interested in membership in a pre-specified birth cohort. Thus, for instance, somebody whose birthday is in February but who had the baby in January might count in one numerator and not in the other.

ASFRs might be reported in either of two ways:

  • As plain numbers, with an implied denominator of 1. This approach is more useful in the context of computing the total fertility rate.
  • As numbers scaled out of 1000, i.e., the number of live births per 1000 women of a given age. This approach is more useful in the context of computing the general fertility rate.

Age-specific fertility rate by birth order

The age-specific fertility rate information could be supplemented with further information, including a breakdown by birth order. Explicitly, the age-specific fertility rate for a given birth order i and age x is defined as the ratio of the number of live births at birth order i to women of age x (see the notes above on operationalization) to the total size of the population of females of age x.

Measurement

Measuring all the ASFRs requires measuring two quantities:

  • Number of live births for each maternal age: This is typically computed by recording all live births, recorded along with the mother's date of birth.
  • The population of females for each age in the childbearing age range: Typically, this appears as part of the full data on the total population and its age-sex structure. Some countries maintain a continuous census, so that reliable information on this is available. Other countries combine a periodic census (conducted every 5 or 10 years) with estimates of population changes based on information about births, deaths, immigration, and emigration. Note that for deaths, immigration, and emigration, it is important to record the age and sex of the person who dies, immigrates, or emigrates in order to maintain an accurate picture of the population's age-sex structure.