A dwelling unit is defined as the living quarters occupied by, or intended for occupancy by, one household. A dwelling unit may be a detached house; a tenement, flat, or apartment in a larger building (apartment house, apartment hotel, or section of a hotel devoted entirely to apartment rather than transient use); or a room in a structure used primarily for business or other nonresidential purposes. It may be a tourist cabin, trailer, railroad car, boat, etc., if occupied by person having no other place of residence.

The number of occupied dwelling units in 1940 is, roughly comparable with the number of private families or homes show in census reports for 1930. Thus the population per occupied unit in 1940 may be compared with the population per private family in 1930 to indicate changes in family size. The 1930 data and corresponding items for 1940 are not comparable, however, in the few tracts where the resident population in institutions or other special types of dwelling places accounts for a large proportion of the total population of the area.