The Total Fertility Rate
Useful as it is to see patterns there comes a time when it would be nice to have a single measure of fertility that captures all the information in the ASFRs – this is the Total Fertility Rate (TFR) – and is a very important measure.
Its importance is due to the fact that it is a measure that is effectively independent of age structure and so measures fertility – the number of births that women give birth to – in a purer way than measures mentioned so far, devoid of confounding factors such as different proportions of women at particular ages. This means that TFRs can be compared internationally without concern about age structure effects. This is very useful and the TFR is the standard way to compare fertility levels internationally.
where: a = 1 - 7 refer to the seven 5yr age groups
It is worth talking through this equation. Quite simply it is constructing each of the seven ASFRs and then summing them. The result is multiplied by five – why? Because the age groups are 5 years wide and each ASFR represents the babies one woman will have at a particular single-year age, say age 30. But another woman aged 31 will be having the same fertility and indeed there will be five woman-years of experiencing that level of fertility – hence the multiplication by 5. The multiplication adjusts the sum of the ASFRs so that they then simulate a cohort, albeit a hypothetical one.
It is possible to use single year age groups, although this is not common. In that case there would be 35 ASFRs to be calculated and summed – but there would be no need for multiplication.
Nowadays the TFR is conventionally expressed as a figure relating to a single woman. For nations it mostly ranges between about 1.5 to 6. Historically it could range up to about 8 but that is unusual today. However for selected communities it is quite possible to have TFRs both higher and lower than the normal range.
The highest recorded and verified TFR is that of the North American Hutterite religious community during the 1920s. This was 9.2 for all women and 12.4 if only married women were used for the denominator. The latter figure is an example of a Total Marital Fertility Rate (TMFR) and has been used as a standard for very high (but verified) fertility. Marital (including cohabiting) fertility is naturally a considerably higher figure than the general TFR because it utilises only women who are in family-forming partnerships. The measure has its uses but it is not so common a figure as the general TFR.