He assumed that the typical human survival curves show: a rapid decrease in survival in the first few years of life, a relatively steady decrease, and then an abrupt decrease near death.

In the absence of data on measures of fertility, they were derived from surveys. In India, demographers derived parity progression ratios from survey data and used exponential model, displaced exponential model, Poisson model and life table approach for birth intervals (Pathak, 1989; Krishnamoorthy, 1989; Pathak and Ram, 1989).

PREDICTION AND ESTIMATION OF MIGRATION

Studies of migration aim at estimation, prediction and explanation of migration. In this context, for the first time Ravenstein published an article in Journal of the Statistical Society in 1885 in which he showed that, as believed by William Farr and many others, migration is not without laws. He developed some well known laws of migration, such as follows, which are still difficult to refute:

• The great body of migrants travel short distances.

• Women outnumber men in short distance migration.

• Migrants move from agricultural areas (places of dispersion) to industrial cities (places of absorption) followed by migration from centres of industrial cities to suburban areas and from remote areas to places of dispersion.

• Each migration current has a counter current with similar characteristics.

• The major causes of migration are economic.