解説・定義

Net cumulative fertility and net reproduction rate. Because of a high infant mortality, the size of a young cohort of men or women will rapidly decline. Many people won't reach the age at reproduction. The cumulative net fertility which counts only births from survivors (f(x)*l(x)) will be much lower than the cumulative (crude) fertility. Also for males the loss of reproduction capacity, due to mortality is more important than for females because male fertility is still high at older ages where survivors are scarce.
In Africa, fertility is big enough to counterbalance the mortality depletion. One can see on figure 27 that before the epidemic female cohorts were already above the reproduction threshold at age 23-24 (cumulative net fertility reaches 1 at age 23-24). The total cumulative net fertility or net reproduction rate is 2.43 at age 50 and over. A net reproduction rate lower than 1 will correspond to a declining population. A net reproduction rate is expressed in terms of daughters per girl or sons per boy. It can be converted in terms of children per girl or children per boy using the observed law that at birth there are about 105 boys for 100 girls.