A structure is defined as a separate building that either has open space on all four sides, or is separated from other structures by dividing walls that extend from ground to roof.

Statistics are presented in terms of the number of housing units rather than the number of residential structures. However, the number of structures for the first two categories may be derived. For 1-unit structures (which include trailers), the number of housing units and the number of structures are the same. For 2- unit structures, the number of housing units is twice the number of structures. For the remaining categories, the number of structures cannot be derived from the data as tabulated.
The categories for number of housing units in the structure in 1960 are not directly comparable with those in 1950, particularly for 1- and 2-unit structures. In the 1950 tract report, units in detached or attached structures were shown separately but those in semidetached structures containing 1 or 2 units were combined into one category. Comparability between 1950 and 1960 data may else be affected by the change in concept from dwelling unit to housing unit.

In order to obtain data on Spanish- and Mexican/Americans for areas of the United States where most of them live, white persons (and white heads of households) of Spanish surname were distinguished separately in five Southwestern States (Arizona, California, Colorado, New Mexico, and Texas). In all other States, Puerto Rican persons (and heads of households) were identified. Puerto Rican comprise persons born in Puerto Rico and persons of native parentage with at least one parent born in Puerto Rico.

Living quarters were enumerated as housing units or group quarters. Occupied living quarters were classified as housing units or group quarters on the basis of information supplied by household members on the Advance Census Report and questions asked by the enumerator where necessary. Identification of vacant housing units was based partly on observation by the enumerator and partly on information obtained from owners, landlords, or neighbors. A house, an apartment or other group of rooms, or a single room is regarded as a housing unit when it is occupied or intended for occupancy as separate living quarters, that is, when the occupants do not live and eat with any other persons in the structure and there is either (1) direct access from the outside or through a common hall or (2) a kitchen or cooking equipment for an exclusive use of the occupants of the unit.

Occupied quarters which do not qualify as housing units are classified as group quarters. They are located most frequently in institutions, hospitals, nurses homes, rooming and boarding houses, military and other types of barracks, college dormitory, fraternity and sorority houses, convents, and monasteries. Group quarters are also located in a house or apartment in which the living quarters are shared by the person in charge and five more persons unrelated to him. Group quarters are not included in the housing inventory, although the count of persons living in them is included in the population figures.

The inventory of housing units includes both vacant and occupied units. Newly constructed vacant units were included in the inventory if construction had reached the point that all the exterior windows and doors were installed and the final usable floors were in place. Dilapidated vacant units were included providing they were still usable as living quarters; they were excluded they were being demolished or if there was positive evidence that they were to be demolished.

Trailers, tents, boats, and railroad cars were included in the housing inventory if they were occupied as housing units. They were excluded if they were vacant, used only for extra sleeping space or vacations, or used only for business.

In 1950, the unit of enumeration was the dwelling unit. Although the definition of the housing unit in 1960 is essential similar to that of the dwelling unit in 1950, the housing unit definition was designed to encompass all private living quarter whereas the dwelling unit definition did not completely cover private living accommodations. The main difference between housing units and dwelling units is as follows: In 1960, separate living quarters consisting of one room with direct access but without separate cooking equipment qualify as a housing unit whether in an apartment house, rooming house, or house converted for the apartment use; in hotels, a single room qualifies as a housing unit if occupied by a person whose usual residence is the hotel or person who has no usual residence elsewhere. In 1950, a one-room unit without cooking equipment qualified as a dwelling unit when located in a regular apartment house or when the room constituted the only living quarters in the structure.

The evidence thus far suggests that using the housing unit concept in 1960 instead of the dwelling unit concept as in 1950 has relatively little effect on the counts for large areas and for the Nation. Any effect which the change in concept may have comparability that can be expected to be greatest in statistics for certain census tracts and blocks. Living quarters classified as housing units in 1960 but which would not have been classified as dwelling units in 1950 tend to be clustered in tracts where many persons live separately in single rooms in hotels, rooming houses, and other light housekeeping quarters. In such areas, the 1960 housing unit count for an individual tract may be higher than the 1950 dwelling unit count even though no units were added by new construction or conversion.