The most important source of data on internal migration in most countries is the national census. These data may be obtained through the census directly by asking a question on migration or may be estimated indirectly.
The usual direct questions on internal migration cover the following items: place of birth, place of last residence, duration of residence in the place of enumeration, and place of residence on a specified data before the census.
Of the various questions asked to obtain direct information on migration, the question on the place of birth is the most widely asked. The United Nations has recommended that this question may be given top priority.
In India, up to 1961, census data on migration were obtained through particulars of the birth place. In 1971, however, an additional question was asked to obtain information on migration, and this question concerned the place of the last usual residence.
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In the 1981 census, in addition to the above questions, information was also obtained about the reasons for migration from place of last residence and duration of residence at the place of enumeration. Information on migration was collected on a sample basis.
Surveys
Direct information on internal migration may also be obtained from sample surveys, which are useful in supplementing the information obtained from a national census.
Such surveys provide information on the characteristics of the migrants, their motives for migration, their attitudes towards migration, etc. Such data are useful in studying the question of internal migration in depth.
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In India, information on internal migration has been collected in several Rounds of the National Sample Survey that is, the Ninth Round, the Eleventh Round, the Twelfth Round, the Thirteenth Round, the Fourteenth Round and the Fifteenth Round.
In 1965, a sample survey on Rural Migration Patterns in Southern Maharashtra was completed by the International Institute for Population Studies, Bombay (the erstwhile Demographic Training and Research Centres, Bombay).
Methods of Measuring Internal Migration
It is customary to classify the techniques used for measuring or estimating internal migration into two categories: (1) direct techniques and (2) indirect techniques.
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The direct techniques are based on data obtained from direct questions asked during a census on the movements of persons.
Questions on the following items provide information for direct estimation of migration movements: the place of birth of the person, the last or previous place of residence of the person, or the duration of stay of a person at the present place of residence.
The indirect techniques of estimating migration do not require special questions; the extent of migration can be estimated from the total counts in a census and the available age- sex distribution of the population.