The data on plumbing facilities were obtained from answers to long-form questionnaire Item 39, which was asked on a sample basis at both occupied and vacant housing units. Complete plumbing facilities include: (1) hot and cold piped water, (2) a flush toilet, and (3) a bathtub or shower. All three facilities must be located inside the house, apartment, or mobile home, but not necessarily in the same room. Housing units are classified as lacking complete plumbing facilities when any of the three facilities is not present.
The 1990 census and Census 2000 data on complete plumbing facilities are not strictly comparable with the 1980 data. Before 1990, complete plumbing facilities were defined as hot and cold piped water, a bathtub or shower, and a flush toilet in the housing unit for the exclusive use of the residents of that unit. In 1990, the Census Bureau dropped the requirement of exclusive use from the definition of complete plumbing facilities. Of the 2.3 million year-round housing units classified in 1980 as lacking complete plumbing for exclusive use, approximately 25 percent of these units had complete plumbing but the facilities also were used by members of another household. From 1940 to 1970, separate and more detailed questions were asked on piped water, bathing, and toilet facilities. Prior to 1990, questions on plumbing facilities were asked on a 100-percent basis. In 1990 and Census 2000, they were asked on a sample basis.