General fertility rate
This article describes a ratio measure. In the standard operationalization, the numerator of the measure is number of live births (view other ratio measures with this numerator) and the denominator of the measure is number of females of childbearing 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.
The general fertility rate of a population over a given time period measures the ratio of the number of live births over that time period to the number of women who are in their childbearing years during that time period, and also divides by the length of the period.
The general fertility rate is operationalized as follows:
- The time period is chosen as one year.
- Childbearing years is operationalized using the age range of either 15-44 or 15-49 (the convention varies). A convention needs to be set regarding the date on which the woman's age is measured within the time period of the year. CHECK THIS: The general convention is to look at women whose age on their birthday anniversary within the year is within the childbearing years age range.
- The ratio is scaled to a denominator of 1000.
Explicitly, the general fertility rate is stated as the ratio of the number of live births in a year for every 1000 women who are in their childbearing years during that year.
Relation with age-specific fertility rates
The general fertility rate is the weighted average of the age-specific fertility rates, weighted by the populations of females at each age.
Distinction between total fertility rate and general fertility rate
Further information: total fertility rate, total fertility rate versus general fertility rate
The reason TFR and GFR are different is because, whereas the TFR is the total of age-specific fertility rates and it weighs the birth cohort for each year with a weight of 1 per birth cohort, the GFR is the average of age-specific fertility rates weighted by the population sizes for the ages.
TFR and GFR would be easily related in the following two cases:
- All the age-specific fertility rates are the same. In this case, the GFR would equal all the equal age-specific fertility rate values, and the TFR would be the product of that value and the number of childbearing years.
- The population size is the same for each age. In this case, the GFR would equal the average of the age-specific fertility rate values, and the TFR would be the sum of the age-specific fertility rate values. In this case again, the TFR would equal the product of the GFR and the number of childbearing years.
Of course, neither case occurs in practice: there is considerable variation in the age structure of the female population (see the population pyramid for more), and considerable variation in the age-specific fertility rates.
Note also that, whereas TFR numbers are scaled to 1 (i.e., they represent actual numeric values), GFR numbers are scaled to a denominator of 1000. If the GFR numbers were not scaled to a denominator of 1000, they would be much smaller than TFR values. Both the above points about the relationship in special cases become valid only after we rescale the GFR and ASFR values to a denominator of 1.