The data on births in one year are often not available, either because there is no vital registration system or because the existing vital registration system does not provide adequate and reliable data.
In such cases, it is possible to measure fertility by the census age distribution to compute the is the ratio of children under age groups 15-19, 20-24, 25-29, 30-34, 35-39 and 40-44 respectively.
Table 8.2 shows how the sex-age adjusted birth-rate is computed, in which the numerator is obtained from the vital registration system. It may be seen that the sex-age adjusted birth rate for India in 1961 was 43.1 per thousand populations.
The data collected through the National Sample Survey indicate that the weights of the age specific fertility rates for India given in column 4 of Table 8.2 differ significantly from those recommended by the United Nations given in Column of the same Table.
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It may also be observed that, even with the actual weights, the value of the sex-1 age adjusted birth rate comes to about 43 per thousand populations.
The logic behind the use of the standard system of weights is I that, even for countries with varying levels of fertility, the relative levels of age specific rates for women in the age groups 15-19 to 40-1 are rather similar.
It should be noted that absolute age specific fertility rates can be either high or low, according to the fertility levels of a country; but the percentage of births at a given age do not differ much.