he Sample Registration System (SRS) is the largest demographic survey in the world covering about 1.4 million hoseholds and 7.1 million population. It has continued to be the main source of information on fertility and mortality indicators both at the state and national levels. It provides reliable annual estimates of birth rate, death rate and other fertility & mortality indicators at the national and sub-national levels. Initiated on a pilot basis by the OFFICE OF THE REGISTRAR GENERAL, INDIA in a few selected states in 1964-65, it became fully operational during 1969-70 with about 3700 sample units. The field investigation consists of continuous enumeration of births and deaths in selected sample units by resident part time enumerators, generally anganwadi workers & teachers, and an independent survey every six months by SRS supervisors. The data obtained by these two independent functionaries are matched. The unmatched and partially matched events are re-verified in the field and thereafter an unduplicated count of births and deaths is obtained. The sample unit in rural areas is a village or a segment of it, if the village population is 2000 or more. In urban areas, the sampling unit is a census enumeration block with population ranging from 750 to 1000. The SRS sample is replaced every ten years based on the latest census frame. It had been a practice to stagger the replacement process over 2-3 years. However, the latest replacement has been carried out in one go. Effective from January 2004, this sample is based on the 2001 Census frame. At present, SRS is operational in 7,597 sample units (4,433 rural and 3,164 urban) spread across all States and Union territories.