If a unit was located in a multiunit structure which was for sale as an entire structure and if the unit was not for rent, it was reported as "held off market." However, if the individual unit was intended to be occupied by the new owner, it was reported as "for sale."

Vacant units rented or sold.-This group consists of year-round vacant units which have been rented or sold but the new renters or owners have not moved in as of the day of enumeration. In the reports published before 1972, this group was limited to units rated "sound or deteriorating."

Vacant units held off the market.-Included in this category are year-round units which were vacant for reasons other than those metioned above: For example, held for occupancy of a caretaker, janitor, and the like; held for settlement of estate; held for personal reasons of the owner; and temporarily occupied by persons who have a usual place of residence elsewhere. In the reports published before 1972, the category was limited to units which were rated "sound or deteriorating."

Seasonal vacant units.-Seasonal housing units are those intended for occupancy only during certain seasons of the year and are found primarily in resort areas. Housing units held for occupancy by migratory labor employed in farm work during the crop season are tabulated as seasonal.

Vacancy rates. In this report several measures are shown for vacant housing units that are on the market for rent or for sale only.

Rental vacancy rate. The percentage relationship of the vacant units for rent to the total rental inventory is termed the rental vacancy rate. It is computed by dividing the number of vacant units for rent by the total rental units. The total rental units consist of the renter-occupied units, vacant units rented but not yet occupied at the time of enumeration, and the vacant units for rent. Vacant units that are seasonal, or held off the market are excluded. In the reports published before 1972, vacant units for rent that were rated as dilapidated were also excluded.

Homeowner vacancy rate.-The percentage relationship between the vacant units for sale and the total homeowner inventory is termed the homeowner vacancy rate. It is computed by dividing the number of vacant units for sale by the total homeowner units. The total homeowner units consist of the owner-occupied units, vacant units sold and awaiting occupancy, and the vacant units for sale. Vacant units that are seasonal, or held off the market are excluded. In the reports published before 1972, vacant units for sale that were rated as dilapidated were also excluded. Rates for vacant units with all plumbing facilities.-The number of vacant rental units with all plumbing facilities is shown in table 2 as a percent of the total rental units; i.e., all renter-occupied units, vacant units rented but not occupied, and all vacant units for rent. Similarly, the number of vacant homeowner units with all plumbing facilities is shown as a percent of total homeowner units; i.e., all owner-occupied units, vacant units sold but not occupied and all vacant units for sale only. These rates differ from those shown in table 3 for units with all plumbing facilities. In table 3, the number of vacant rental units with all plumbing facilities is divided by the number of rental units with all plumbing facilities, occupied and vacant. Similarly, the number of vacant units for sale with all plumbing facilities is shown as a percent of homeowner units with all plumbing facilities.

Vacancy rates by selected characteristics.-Beginning with Series H-111 Report No. 55, additional ratios are given which express the relationship between subclasses of vacant housing units and rental and homeowner units that have a common characteristic.

Selected vacancy rates in rental units are computed by dividing the number of vacant for-rent units in the specified category by the total rental units in that group. The total rental units consist of the renter-occupied units and the vacant for-rent units with the common characteristic. Similarly, vacancy rates in specific types of homeowner units are computed by dividing the total number of vacancies for sale in the selected group by the sum of the total owner-occupied and vacant for-sale units in the same group.

Tenure.-A unit is owner occupied if the owner or co-owner lives in the unit, even if it is mortgaged or not fully paid for. A cooperative or condominium unit is "owner occupied" only if the owner or co-owner lives in it. All other occupied units are classified as "renter occupied," including units rented for cash rent and those occupied without payment of cash rent.

Number of rooms.-All rooms which are used, or are suitable for use, as living quarters were counted in determining the number of rooms in the housing unit. Included are kitchens, bedrooms, dining rooms, living rooms, and permanently enclosed porches that are suitable for year-round use; and finished basement or attic rooms, recreation rooms, suitable for use as living quarters. A kitchenette or half-room which is partitioned off from floor to ceiling was counted as a separate room, but a combined kitchenette and dinette separated only by shelves or cabinets was counted as only one room. Not counted as rooms were bathrooms, strip or pullman kitchens, halls or foyers, acloves, pantries, laundries, closets or storage space, and unfinished basement or attic rooms not suitable for living quarters.

The median number of rooms is the theoretical value which divides the housing units equally, one-half having more rooms and one-half having fewer rooms than the median. In the computation of the median, a continuous distribution was assumed. For example, when the median was in the three-room group, the lower and upper limits were assumed to be 2.5 and 3.5, respectively. Tenths were used in the computation of the median to permit compari

sons.

Number of bedrooms. -Only rooms intended primarily to be slept in were counted as bedrooms. A bed-living room or a den intended primarily for pruposes other than sleeping was not counted as a bedroom. A one-room apartment, therefore, was reported as having no bedroom. Number of housing units in structure.-A structure either stands by itself or has vertical walls dividing it from all other structures. The count of housing units in a structure is the total number of units in the structure, including both occupied and vacant units. A structure may be detached, attached, or semidetached. A detached structure has open space on all four sides; an attached structure is one of a row of three or more adjoining structures, or is a structure attached to a nonresidential structure; while a semidetached structure is one of two adjoining residential structures, each with open space on the remaining three sides. In apartment developments, each building with open space on all sides is considered a separate structure. In the tabulations, occupied trailers are included in the category "one housing unit" in structure.

The statistics reflect the number of housing units in the structure in which they are located, rather than the number of residential structures. In the quarterly surveys, data were obtained on the number of housing units in the structure, regardless of the type of structure (detached, attached, or semidetached) in which the unit was located.

Year structure built.-"Year built" refers to the date the original construction of the structure was completed, and not to any later remodeling, addition, or conversion. The figures on the number of units built during a given period relate to the number of units in existence at the time of enumeration. For occupied trailers, "model year" is the year built.

Duration of vacancy.-The length of time a housing unit was vacant was computed from the day the unit became vacant until the day of enumeration. The data, therefore, do not provide a direct measure of the total length of time units remain vacant. For newly constructed units, the duration of vacancy represents the time period since the date when the unit was considered a vacant housing unit, that is, when construction had reached the point that all exterior windows and doors were installed and final usable floors were in place.

The time intervals used in the tables represent full months, calculated from a date in the month to the same date the following month. For example, if the unit became vacant on October 29 and was still vacant on the day of enumeration, December 20, the time reported would be “1 up to 2 months," meaning that the unit had been vacant for more than 1 month but less than 2 months. Or if the unit became vacant on November 25 and was still vacant on December 20, the time reported would be "less than 1 month."

Plumbing facilities.-Housing units "with all facilities" consist of those which have: Both a flush toilet and a bathtub or shower inside the structure for the exclusive use of the intended occupants of the unit, and hot running water.

Units "lacking facilities" consist of those which lack one or more of the plumbing facilities or which lack exclusive use of these facilities.

Facilities were considered "inside the structure" if they were located within the housing unit or elsewhere in the structure. Facilities are for exclusive use if they are used, or intended for use, by only the occupants of one housing unit.

A housing unit was considered as having hot running water whether it was available the year round or only part of the time. For example, hot running water may be available only during the heating season or at various times during the week.

Contract rent.-For renter-occupied units, the contract rent is the monthly rent agreed upon regardless of any furnishings, utilities, or services that may be included. For vacant units, rent is the amount asked for the unit at the time of enumeration; the amount may differ from the rent contracted for when the unit is occupied. As in the 1970 census, the statistics exclude rents for renter occupied and vacant for rent single family structures on places of 10 acres or more. The rent statistics in the reports for 1962 to 1972 are based on the 1960 definition; the data exclude rents for renter-occupied farm housing units in rural territory and rents for vacant units on places of 10 acres or more in rural

areas.

The median monthly rent is the rent which divides the series into two equal parts, one-half of the units with rents higher than the median and the other half with rents lower than the median. In the computation of the median, a continuous distribution was assumed; and the limits of the class intervals were assumed to stand at the midpoints of the 1-dollar interval between the end of one of the rent groups and the beginning of the next. For example, the limits of the interval designated $60 to $69 were assumed to be $59.50 and $69.50.

Inclusion of utilities in rent. The utilities included in the inquiry were heat, light, cooking fuel, and water. The statistics reflect whether all or not all of these utilities are provided for in the amount of rent asked at the time of enumeration, not what could be provided for more or less rent. Data on the inclusion of utilities are limited to the vacant units for which rent is reported. No data on utilities are collected for renter-occupied units.

Value.-Value is the respondent's estimate of how much the property would sell for on the current market. For vacant units, value is the sale price asked for the property at the time of enumeration and may differ from the price at which the property is sold. Statistics on value for owner-occupied units and sale price asked for vacancies for sale are limited to units in 1-housing unit structures, without business or medical office, and with only one housing unit on the property. Statistics on value also exclude occupied mobile homes or trailers. The statistics in this report are for owner-occupied and vacant for-sale units which are located on places of less than 10 acres; thus units on places of 10 acres or more are excluded. The statistics in the reports for 1962 to 1972 are based on the 1960 definition, and exclude units in rural areas on places of 10 acres or more.

The median value or sale price is the amount which divides the series into two equal parts, one-half of the units with values higher than the median and the other half with values lower than the median. The median was computed on the basis of more detailed tabulation groups than are shown in the tables and was rounded to the nearest hundred dollars.

Source of data.-The estimates presented in this report are based on data obtained from two surveys conducted by the Bureau of the Census. Data concerning vacancy rates and tenure of occupied housing units are from the monthly sample of the Current Population Survey (CPS). Characteristics of occupied housing units are from the Quarterly Household Survey (QHS).

The Current Population Survey sample is spread over 461 areas comprising 923 counties and independent cities with coverage in each of the 50 States and the District of Columbia. Approximately 47,000 occupied households are designated for interview each month; of this number 2,000 occupied units, on the average, are visited but interviews are not obtained because the occupants are not found at home after repeated calls or are unavailable for some other reason. In addition to the 47,000, there are also about 8,000 sample units in an average month which are visited but are found to be vacant or otherwise not to be interviewed.' Data on vacancy rates and tenure of occupied units for the year 1973 are averaged over the 12 months of the year.

The data concerning characteristics of occupied housing units are from the Quarterly Household Survey (QHS). The annual figures in this report are based on the average of four quarterly estimates produced from interviews conducted in January, April, July, and October of 1971, 1972, and 1973. During this period of time, January 1971 to October 1973, the QHS sample was redesigned; three sample designs were involved. For the four quarters of 1971 and 1972 the QHS was spread over 235 areas, comprising 484 counties and independent cities. For January and April 1973, the QHS sample was in 266 areas, comprising 578 counties and independent cities. For July and October 1973, the areas were reduced to 103, comprising 296 counties and independent cities.

The estimates of occupied housing units for this report were based on approximately one-third of the units designated for the full QHS samples within each of the areas. Estimates for a full QHS sample were based on interviews conducted in a subsample of households selected from a larger sample within the areas; the subsample was derived by eliminating a random subset of owner-occupied and low income households identified in the larger sample. The samples varied in size over the surveys, reflecting the changes in the sample designs. For the 1971-1972 and the January-April 1973 interview periods, the larger samples (before subsampling) amounted to about 16,000 designated units; about 7,000 units were designated for the JulyOctober 1973 interview period. Subsamples of about 11,500, 13,000, and 6,000 interviewed households were selected for the respective full QHS samples. The occupied housing unit data used for this report are based on about one-third of these units. Units with family incomes below $5,000 for 1971 and 1972 and $4,000 for 1973 were treated as low income households for the purpose of identifying the smaller samples.

In addition to the interviewed households, for each of the quarters a set of units, amounting to about 5 percent of the larger sample of occupied units, were visited but interviews were not obtained because the occupants were not found at home after repeated calls or were unavailable for some other reason. There were also additional units, amounting to about 15 percent of the larger sample, which were visited but found to be vacant or otherwise not to be interviewed. Data for these units were not included in the figures for this report.

The estimating procedures used for the QHS involve the inflation of the weighted sample results to independent estimates of the number of owner-occupied and renteroccupied housing units by urban and rural residence based on data from the Current Population Survey (CPS). These independent estimates have a lower sampling error due to the larger sample size and because other current independent estimates of the population of the United States are employed in the CPS estimation procedure.

Reliability of the estimates. Since the estimates are based on a sample, they may differ somewhat from figures obtained if a complete census had been taken using the same schedules, instructions, and enumerators. As in any survey work, the results are subject to errors of response and of reporting as well as being subject to sampling variability.

The sampling error is primarily a measure of sampling variability, that is, of the variations that occur by chance because a sample rather than the whole of the population is surveyed. As calculated for this report, the sampling error also partially measures the effect of response and enumeration errors but does not measure any systematic biases in the data. The chances are about 68 out of 100 that an estimate from the sample would differ from a complete census figure by less than the sampling error. The chances are about 95 out of 100 that the difference would be less than twice the sampling error.

The figures in table A are sampling errors of selected annual vacancy rates. Annual vacancy rates are subject to smaller variation than quarterly rates. The sampling error for the annual average rates can be approximated by applying a factor of 0.6 to the sampling error of the quarterly vacancy rates.

Comparison of characteristics of vacant and occupied housing units for 1973 with previous years (Series H-111, Nos. 1 to 72-5) reveals that many of the differences are small. A small difference, particularly when it is based on a small subclass of housing units, should be interpreted with care. As can be seen from tables C and D, the sampling variability in such cases may be large relative to the difference.

Tables C and D show approximations to the sampling errors for estimated percentages of vacant and occupied housing units. In order to derive sampling errors applicable to a wide variety of items that could be prepared at a moderate cost, a number of approximations were required. As a result, the table provides an indication of the order of magnitude of the sampling errors rather than the precise

sampling error for any specific item. Sampling errors are shown for estimates of the percent of housing units having a specific characteristic when the base of the percent is all occupied housing units, owner-occupied housing units, or renter-occupied housing units.

Illustration of the use of the tables of sampling errors.-The rental vacancy rate for the year 1972 is estimated to be 5.8 percent. Table A shows the sampling error of this estimate to be approximately 0.1 percent. The chances are 68 out of 100 that the estimated 5.8 percent would be within 0.1 percent of a complete census figures, and chances are 95 out of 100 that the estimate would be within 0.2 percent of a

complete figure. That is, the 95 percent confidence interval would be between 5.6 and 6.0 percent.

Table 19 of this report shows that 19 percent of the renter-occupied units were built in 1965 or later. Table D shows the sampling error of 19 percent on a base of all renter-occupied units to be approximately 0.5 percent. Consequently, chances are 68 out of 100 that the estimated 19 percent would be within 0.5 percent of a complete census figure, and chances are 95 out of 100 that the estimate would be within 1.0 percent of a census figure. That is, the 95 percent confidence interval would be between 18 and 20 percent.