The "NCHS Definitions" section of the National Center for Health Statistics website provides the following information about classifications for place of residence for the survey:

The NCHS Definitions webpage further notes that "the largest city in an MSA is always a central city." Households coded as "MSA, not in Central City" are located in any part of an MSA that is not part of the central city itself. Households coded as "Not in MSA" are in all other places in the country except MSA's.

Only households in areas that were classified as "rural" were at risk of being classified as farms. For 1969-1981, a rural household was classified as a farm if: 1) it was located on 10+ acres that yielded $50+ from the sale of crops, livestock, and other farm products; or, 2) it was located on less than 10 acres that yielded $250+ from the sale of crops, livestock, and other farm products. Beginning in 1982, a farm was any rural household that yielded $1,000+ from the sale of crops, livestock, and other farm products. These definitions of a farm basically matched those adopted in the 1960 and 1970 censuses and in the 1980-2000 censuses, respectively. Survey respondents supplied the information whether the size of the place that the household owned or rented was 10 or more acres (for 1969-1981) and about whether the sale of farm products during the past 12 months reached a specified threshold ($50, $250, or $1000).

Non-interview households are those in which the dwelling was unoccupied or in which the residents were either absent or refused to participate in the survey.