Total fertility rate versus general fertility rate
Key differences
Difference type | For TFR | For GFR |
---|---|---|
essential purpose | describing how many children a female would have if at each age she behaved like the average of the women currently at that age | describing how many children have been added in a particular year, relative to the number of women |
relation with age-specific fertility rates | it is a total | it is a weighted average |
relation with age-specific fertility rates, weighting used | we use a weighting of 1, i.e., we just add up the ASFR values | we weight by the population of women of that age |
scaling used to express the value | scaled to a denominator of 1, i.e., we report the TFR per woman | scaled to a denominator of 1000, i.e., we report the GFR per 1000 women. This is simply because the GFR value per woman would be too small, and we generally prefer to use a scaling where we get reasonably large and easily comparable numeric values. |
Cases where they convey equivalent information
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.