Définition
If a (fictional) generation of 100 women were supposed not to be subject to the effect of mortality and were to have, at each age (between 15 and 50) the fertility observed in year n, then these 100 women would give birth in the course of their lives to a number of girls which is, by definition, the gross reproduction rate (for year n). This rate is in fact a fertility index for year n, irrespective of the actual age structure, and therefore enables comparisons in time and space (unlike the birth rate which depends both on fertility and on the structure of the population).
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.