India - Employment and Unemployment Survey: NSS 50th Round, Schedule 10, July 1993 - June 1994

In the rural sector, each district is treated as a separate stratum. However, if the census population of the districts is greater than or equal to 1.8 million or 2 million (depending upon whether 1981 or 1991 census frame is used for selection of villages), the district has been split into two or more strata, by grouping contiguous tehsils to form strata. In Gujarat, in the cases of districts extending over more than one region the part of a district falling in each region constitues a separate strsatum. In the urban sector, strata are formed within NSS region on the basis of the population size class of towns.
See INSTRUCTIONS TO FIELD STAFF : VOLUME- I (find attached as external resource attachment)