Except for the instances noted below, the concepts and definitions used in the quarterly surveys are identical with those used in the 1960 census.

Standard metropolitan statistical area (SMSA).-Except in New England, an SMSA is county or group of contiguous counties which contains at least one city of 50,000 inhabitants or more, or "twin cities" with a combined population of at least 50,000, the smaller of which must have at least 15,000. In addition to the county, or counties, containing such a city or cities, contiguous counties are included in an SMSA if, according to certain criteria, they are essentially metropolitan in character and are socially and economically integrated with the central city. In New England, SMSA's consist of towns and cities, rather than counties. A more detailed discussion of the criteria used to define SMSA's is given in 1960 Census of Housing, Volume I, States and Small Areas.

The statistics for inside SMSA's and outside SMSA's presented for the years 1962 to 1971 relate to the areas (212) designated SMSA's at the time of the April 1960 census. Prior to 1962, the statistics related to the areas (168) designated SMA's at the time of the 1950 census. For a list of the 212 SMSA's and their constituent parts, refer to the individual chapters (or Series HC(1) reports) in the 1960 Census of Housing, Volume 1, States and Small Areas; or to the 1960 Census of Population, Volume I, Number of Inhabitants. For a list of the 168 areas and their constituent parts, refer to the individual chapters (or bulletins) in the 1950 Census of Housing, Volume I, General Characteristics; or to the 1950 Census of Population, Volume I, Number of Inhabitants, or Volume II, Characteristics of the Population, Part I, United States Summary.

Housing unit.-- Beginning with the second quarter 1960, the 1960 census housing unit concept was used to enumerate separate living quarters. Prior to that time, separate living quarters were enumerated according to the 1950 dwelling unit definition. The major difference between the two is not in content but in sharpening of the criteria by which separate units are identified. It is expected that the housing unit concept will yield. better coverage of both occupied and vacant living quarters than the dwelling unit concept. Although no statistical measure of the effect of the change is available at this time, it is believed to be too small to affect the vacancy rates.

A group of rooms or a single room is regarded as a housing unit when it is occupied as separate living quarters, that is, when the oc

cupants 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 the exclusive use of the occupants.

Vacant living quarters intended for occupancy as separate living quarters and those that meet the physical criteria defined above are separate housing units. Because it is customary in some localities for occupants to furnish their own cooking equipment, vacant units without a kitchen or cooking equipment are considered to have such equipment if the last occupants had cooking equipment.

Living quarters of the following types are excluded from the housing unit inventory: Dormitories, bunkhouses, and barracks; quarters in predominantly transient hotels, motels, and the like, except those occupied by persons who consider the hotel their usual place of residence; quarters in institutions, general hospitals, and military installations except those occupied by staff members or resident employees who have separate living arrangements.

Quarters for roomers or boarders having shared living arrangements are excluded from the housing inventory if there are five or more such rooms; they are combined as one housing unit if there are four or fewer such rooms.

Trailers, tents, and boats are excluded if vacant, used for business, or used for extra sleeping space or vacations.

Dwelling unit, 1950.-- The 1950 dwelling unit was defined as follows: A group of rooms occupied as separate living quarters was a dwelling unit if it had separate cooking equipment or if it constituted the only living quarters in the structure. Each apartment in a regular apartment house was a dwelling unit even though it may not have had separate cooking equipment. Apartments in residential hotels were dwelling units if they had separate cooking equipment or consisted of

two or more rooms.

Vacant quarters intended for occupancy as separate living quarters and met the physical criteria defined above were separate dwelling units. Vacant units were considered as having cooking equipment if they were equipped with such equipment or if the last occupants had cooking equipment.

In general, living quarters of the types excluded from the housing unit inventory were excluded from the dwelling unit inventory.