There are three important sources of information on fertility: (1) The Vital Registration or the Civil Registration System; (2) The National Periodic Census; and (3) Sample Fertility Surveys.
1. From the vital registration system, information is available on the number of registered births in each calendar year.
Along with this information, ancillary information on the following items is also available from this system: date of birth, sex of the baby, order of birth, religion of parents, their occupation, their place of residence, etc.
The Demographic Year Book of the United Nations, published each year, contains information on the registered live births for various countries cross-tabulated with the age of the mother, sex of the baby, crude birth rates for rural and urban areas etc.
Natality (fertility) was the special topic for the Year Books of 1945-50, 1959, 1969 and 1975 and, therefore, these Year Books contained a great deal of additional information on fertility for different countries.
2. In the national periodic census of a country direct question on “the number of children ever born” is asked of ever-married women and the answers to this question provide the basis for information on fertility.
In some countries, where the birth registration system is inadequate, a question on the number of births during the previous year is asked during a national periodic census. Such a question was asked in the 1971 as well as in the 1981 Census of India.
3. Another important source of information on fertility is the Sample Fertility Survey. Such a survey provides fairly reliable information of the fertility of any group, and has several advantages over the vital registration system or the census.
The main advantage being that it is possible to collect in such a survey a great deal of the required information for an analysis of fertility from different angles, such as information on age at marriage, reproductive history, knowledge of, attitude to, and practice concerning family planning, etc.
For this reasons, in many developed countries, despite a reliable and adequate vital registration system and availability of information on the number of children ever born in a periodic census, special sample fertility surveys are conducted. Some of these surveys have made a valuable contribution to the development of fertility analysis.
The type of data available from each of these sources may be summarised as follows:
1. Vital Registration:
(a) The number of registered System: births, usually in one calendar year.
(a) The number of children ever born.
(b) Census age distribution.
(c) The number of births during the last twelve months.
3. Sample Fertility:
(a) The number of children Survey: ever born.
(b) The number of births during the last twelve months.