The Total Fertility Rate (cont.)
More on Tempo and Quantum
Two terms are associated with the TFR – Tempo and Quantum – and both these are associated with the variability of the measure.
Quantum relates to the real value of the TFR – the real level of fertility over a substantial time. If TFR is 4.2 then one expects that over a reproductive lifetime 4.2 births will be produced per woman on average. 4.2 is the quantum of fertility in that case and if fertility is truly rising (over a longish period) then one expects more than 4.2 births to be the result.
But we know that the TFR can fluctuate considerably from year to year – following economic trends, strife etc. If the TFR rises for a few years but then drops back the quantum of fertility may not be really changing – it is just the timing of births that has produced an apparent and temporary change. This is a Tempo effect and, as in Figure 5 for France, we know that the period TFR suffers from this effect.
In fact if couples decide to space births further apart – perhaps because of economic conditions – then immediately the period TFR will begin to fall because the pace of childbearing is not being kept up. However, a few years later they have the same number of children that they planned to have – so the quantum is the same.
Although the TFR is very widely used there are times when demographers want to deal with measures that are free from tempo effects and therefore more stable. This inevitably involves looking at cohort fertility.
PAPP103_S09 deals with Quantum and Tempo effects in more detail.