Définition
We will consider a (fictional) generation of 100 women from birth through to the end of the childbearing period of their life: we will suppose that this generation is subject to the mortality observed during year n, and that at each age (from 15 upwards) it has the fertility observed in year n. This generation (the number of which will have been reduced by mortality to less than 100 by the age of 50) would give birth to a number of daughters which is, by definition, the net reproduction rate (for year n).
This index shows the extent to which one generation would be replaced by the next under the mortality and fertility conditions of the year in question: if the rate is greater than 100, the generation would be replaced by a larger generation and the population would increase; if it is less than 100, the generation would tend to be replaced by a smaller generation and the population would decrease.
Remarque
It should not be forgotten that the rates used in the calculation are those observed over a given year for the whole female population (composed of several generations) and therefore do not represent the rates for an actual generation of women. It is probable that no actual generation will have the observed rates at each age. Reproduction rates therefore serve only to give an overview of the demographic situation over a given year, without being able to draw any certain conclusions from them as to the future of the population.