"In 1990, only 1 percent of our homes lacked complete plumbing facilities. But, things were much different in 1940, when nearly half lacked complete plumbing. Then, about ten States had rates approaching or exceeding 70 percent. In succeeding decades, the proportion of homes lacking complete plumbing dropped dramatically, falling to about one-third in 1950 and one- sixth in 1960. It is interesting to note the States with the lowest percent- age of such homes in 1940 were higher than Alaska, which topped the 1990 list.

Complete plumbing facilities are defined as hot and cold piped water, a bath- tub or shower, and a flush toilet. In earlier censuses, these facilities must have been for exclusive use of a housing unit's inhabitants; this requirement was dropped in 1990"

https://www.census.gov/data/tables/time-series/dec/coh-plumb...