Going back to source an attempt to trade the differences in findings to different methodologies of IRS and NRS

NRS defines its universe as the resident urban population of India aged 15 years and above.
The study covered all of urban India, excluding J&K due to law and order situation posing problems for satisfactory conduct of fieldwork, and the offshore territories of Lakshwadeep and Andaman & Nicobar islands. In addition it covered rural Kerala. The All India sample size was 135,430 of which the urban sample was 131,568 spread across 504 towns. The urban sample included a SEC A1 booster of 9,696 interviews in the top 23 metros.

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