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.

This category includes people who indicated their race as "American Indian," entered the name of an Indian tribe, or reported such entries as Canadian Indian, French American Indian, or Spanish American Indian.

Respondents who identified themselves as American Indian were asked to report their enrolled or principal tribe. Therefore, tribal data in tabulations reflect the written entries reported on the questionnaires. Some of the entries (for example, Iroquois, Sioux, Colorado River, and Flathead) represent nations or reservations. The information on tribe is based on self-identification and therefore does not reflect any designation of federally or state-recognized tribe. Information on American Indian tribes is presented in summary files. The information for Census 2000 is derived from the American Indian Tribal Classification List for the 1990 census that was updated based on a December 1997, Federal Register Notice, entitled " Indian Entities Recognized and Eligible to Receive Service From the United States Bureau of Indian Affairs," Department of the Interior, Bureau of Indian Affairs, issued by the Office of Management and Budget.

This category includes written responses of Eskimos, Aleuts, and Alaska Indians as well as entries such as Arctic Slope, Inupiat, Yupik, Alutiiq, Egegik, and Pribilovian. The Alaska tribes are the Alaskan Athabascan, Tlingit, and Haida. The information for Census 2000 is based on the American Indian Tribal Classification List for the 1990 census, which was expanded to list the individual Alaska Native Villages when provided as a written response for race.