India - Village Facilities Survey , July 2002-December 2002, NSS 58th Round

In Rural sector: Two special strata were formed at the State/ UT level on the basis of the population census of 1991, viz. Stratum 1: all FSUs with population between 0 and 50, and Stratum 2: FSUs with population more than 15,000
Special stratum 1 was formed if at least 50 such FSUs were found in a State/UT.
Similarly, special stratum 2 was formed if at least 4 such FSUs were found in a State/UT. Otherwise, such FSUs were merged with the general strata.
From the remaining FSUs (not covered under strata 1 & 2) general strata (hereafter, stratum will refer to general stratum unless otherwise mentioned) were formed and numbered 3, 4, 5, etc. (even if no special strata had been formed). Each district of a State/UT was normally treated as a separate stratum. However, if the provisional population of the district was greater than or equal to 2.5 million as per Census 2001, the district was divided into two or more strata with more or less equal populations as per Census 1991 by grouping contiguous tehsils. However, in Gujarat, some districts
were not wholly contained in an NSS region. In such cases, the part of the district falling in an NSS region constituted a separate stratum.