Fertility Rate

The term fertility, as used in the social sciences, refers to the actual births of children, and the fertility rate attempts to gauge the number of children being born into a population. The simplest of these rates is the crude birth rate (CBR), which is the number of live births in a year divided by the total midyear population. It is usually multiplied by 1,000 to reduce the number of decimals. The CBR is “crude” because it does not take into account which groups of people in the population were actually likely to have births and ignores the age structure of the population, which greatly affects how many live births can be expected in a given year. Thus, the CBR (which is sometimes called simply “the birth rate”) can mask significant differences in actual reproductive behavior between two populations and, on the other hand, can imply differences that do not really exist.

The general fertility rate (GFR) uses information about the age and sex structure of a population to be more specific about who actually was at risk of having the births recorded in a given year. The GFR (which is sometimes simply called “the fertility rate”) is the total number of births in a year divided by the number of women of childbearing age (typically 15 through 44 yrs. [years]).

One of the more precise ways of measuring fertility is the age-specific fertility rate (ASFR). This requires a rather complete set of data: births according to the age of the mother and a distribution of the total population by age and sex. The ASFR is the number of births occurring in a year to women within a given age group per 1,000 women of that age (usually given in 5-yr age groups). ASFRs require that comparisons of fertility be done on an age-by-age basis, but demographers have also devised a method for combining ASFRs into a single fertility index covering all ages. This is called the total fertility rate (TFR), and it is the sum of the ASFRs over all ages. Note that if the ASFRs are calculated for 5-yr age groups, then the sum of ASFRs must be multiplied by 5, whereas if data by single year of age are available, that adjustment is not required. The TFR can be readily compared from one population to another because it takes into account the differences in age structure and its interpretation is simple and straightforward. The TFR is an estimate of the average number of children born to each woman in her lifetime, assuming that current birth rates remain constant and none of the women dies before reaching the end of the childbearing years. A rough estimate of the TFR (measured per 1,000 women) can be obtained by multiplying the GFR by 30 or by multiplying the CBR by 4.5 and then again by 30. Although there are several other indices that are used as fertility rates, the CBR, GFR, and TFR are the most common.