<P> The TFR is, therefore, a measure of the fertility of an imaginary woman who passes through her reproductive life subject to all the age - specific fertility rates for ages 15--49 that were recorded for a given population in a given year . The TFR represents the average number of children a woman would potentially have, were she to fast - forward through all her childbearing years in a single year, under all the age - specific fertility rates for that year . In other words, this rate is the number of children a woman would have if she was subject to prevailing fertility rates at all ages from a single given year, and survives throughout all her childbearing years . </P> <P> An alternative fertility measure is the net reproduction rate (NRR), which measures the number of daughters a woman would have in her lifetime if she were subject to prevailing age - specific fertility and mortality rates in the given year . When the NRR is exactly one, then each generation of women is exactly reproducing itself . The NRR is less widely used than the TFR, and the United Nations stopped reporting NRR data for member nations after 1998 . But the NRR is particularly relevant where the number of male babies born is very high due to gender imbalance and sex selection . This is a significant factor in world population, due to the high level of gender imbalance in the very populous nations of China and India . The gross reproduction rate (GRR), is the same as the NRR, except that--like the TFR--it ignores life expectancy . </P> <P> The TFR (or TPFR--total period fertility rate) is a better index of fertility than the crude birth rate (annual number of births per thousand population) because it is independent of the age structure of the population, but it is a poorer estimate of actual completed family size than the total cohort fertility rate, which is obtained by summing the age - specific fertility rates that actually applied to each cohort as they aged through time . In particular, the TFR does not necessarily predict how many children young women now will eventually have, as their fertility rates in years to come may change from those of older women now . However, the TFR is a reasonable summary of current fertility levels . </P> <P> The TPFR (total period fertility rate) is affected by a tempo effect--if age of childbearing increases (and life cycle fertility in unchanged) then while the age of childbearing is increasing, TPFR will be lower (because the births are occurring later), and then the age of childbearing stops increasing, the TPFR will increase (due to the deferred births occurring in the later period) even though the life cycle fertility has been unchanged . In other words, the TPFR is a misleading measure of life cycle fertility when childbearing age is changing, due to this statistical artifact . This is a significant factor in some countries, such as the Czech Republic and Spain in the 1990s . Some measures seek to adjust for this timing effect to gain a better measure of life - cycle fertility . </P>

Difference between total fertility rate and crude birth rate
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