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How is the Total Fertility Rate (TFR) calculated?

The Total Fertility Rate (TFR) is a standard demographic indicator used internationally to estimate the average number of children that a woman would have over her childbearing years (i.e. age 15-49), based on current birth trends.

It is calculated by adding up the average number of births per woman across five-year age groups (i.e. age-specific fertility rates, or ASFR). The specific formula to calculate TFR is:



What this means is that there are only two key components that affect TFR:
The number of births for each age group of women (in the numerator), and
The number of women in each age group (in the denominator).

Does an increase in births mean that TFR will definitely go up?

An increase in the number of births does not necessarily lead to an increase in TFR. Whether TFR will increase depends not only on the number of births (numerator), but also on the number of women in the childbearing age groups (denominator). Thus, even if there are more births, TFR may not increase if there are also more women in the childbearing age groups. 

We observed this phenomenon in 2015, where the number of Singapore Citizen births increased to 33,725 (from 33,193 in 2014), yet TFR declined slightly to 1.24 (from 1.25 in 2014). While there were more babies, the number of women entering the peak child-bearing age groups (25-39 years) had also risen. These women were born in the decade starting from 1988, and many of them are children of the Baby Boomer generation. The oldest of this cohort was just 27 years old in 2015, and hence may not have started having children yet. 

Does the number of marriages or age at marriage/ birth affect TFR?

From the formula above, we can see that the number of marriages does not directly affect TFR, since it is not in the numerator or the denominator. 

Similarly, TFR is not directly affected by the age at first marriage or age at first birth. What is more important is the number of births women have on average, regardless of the age at which they get married or have children. However, there may be indirect effects, as data in Singapore shows that women who get married and start having children earlier tend to have more children over their lifetime. 

 

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