Total fertility rate versus general fertility rate
|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.