Total fertility rate
The total fertility rate (TFR), also called the period fertility rate or period fertility of a population is defined in the following equivalent ways:
- It is the sum of the age-specific fertility rates for females at all ages. Note that only birth cohorts of females in their childbearing years are expceted to have age-specific fertility rates that are noticeably different from zero, so we can also define TFR as the sum of age-specific fertility rates for all ages that fall within the defined span for childbearing years.
- It is the fertility that a female would have if, at every age, she had the age-specific fertility rate observed for the current year (rather than the year when she'd actually be that age).
We use the term period fertility to describe the TFR because it is a form of period analysis: we are aggregating over people of different ages, all in a given period of time. This is in contrast with cohort analysis, where we are looking at people who share a common temporal experience (such as birth, school attendance, or marriage) and study their behavior over their whole lifetime or a long part thereof.