Evaluators may derive ASFRs from several sources. When evaluators estimate ASFR from vital statistics, they use population projections or estimates of the number of women in each age group 15-49 for the denominator in the rate. When using population censuses or surveys, evaluators obtain both the numerator and denominator of the rate from the census or survey. Estimates from censuses are derived from questions on births during a specified period preceding the census (usually 12 months), while survey estimates may be derived either from questions on births within a specified prior period or from partial or complete birth histories.

A simpler, although less precise, procedure for computing the denominator of the rate is to take the average of the number of women in each age group during the reference period covered by the measure (i.e., the average of the numbers of women in each age group at the beginning and end of the reference period).

Reference periods of more than one year are frequently used to compute ASFRs from survey data, the rationale being to decrease sampling variability associated with relatively small numbers of annual births occurring to women in single or five-year age groups and the distorting effects of reference period reporting errors. Various analyses of DHS fertility data, for example, alternately use the three- or five-year period prior to the survey in calculating ASFRs (Arnold and Blanc, 1989; Lutz, 1990). When multiple years are used for computational purposes, average annual rates are normally presented.

ASFRs are sometimes presented for different groups of women; for example, ASFRs are for women currently married or in union and for all women of reproductive age in DHS country reports. In societies where fertility is largely confined to marriage, ASFRs for women currently married or in union will provide more or less complete coverage of recent fertility. Where a large share of fertility occurs outside of recognized unions, however, the restriction of the ASFR to currently married women will result in an under estimate of the level of current fertility.

The ASFR is of particular interest in countries, cities, or districts with adolescent RH interventions designed to reduce unintended pregnancy. Although the ASFR is rarely used as an outcome measure in evaluating such programs (due to the human, financial, and logistic resources needed to collect the data), it is a variable that program administrators and policy makers track over time as a macro-level indicator of program effectiveness combined with non-program influences.

To address this problem, evaluators may calculate ASFRs only for women who were continuously married or in union during the reference period of the measure. The resulting measure is known as the marital age-specific fertility rate (MASFR). However, to calculate this measure, evaluators require data on duration of marriage or marriage histories. In actual practice, MASFRs are more often approximated by calculating ASFRs for women married or in union at the time of a survey, although evaluators should recognize that this figure only approximates the MASFR because women who are married or in union at the time of a given survey may not have been continuously married or in union over the entire reference period of the measure (e.g., for the three to five years prior to the survey).