A CONTROLLED HOUSING UNIT METHOD FOR SMALL AREA POPULATION ESTIMATES

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1 Constance C. Blackmon Data Center Director Atlanta Regional Commission 230 Peachtree Street, NW Atlanta, Georgia A CONTROLLED HOUSING UNIT METHOD FOR SMALL AREA POPULATION ESTIMATES ABSTRACT. The Atlanta Regional Commission prepares annual estimates of population and housing for 243 census tracts and 53 local governments in a seven-county region. The estimates are produced by a two-step process which involves three independent methods. First, the total population estimate for the seven-county Atlanta Region is derived using two different methods: the ratio method and multiple linear regression analysis. Results of the two methods are averaged to produce the regional control total for census tracts and component jurisdictions of the region. Separate estimates for each census tract and jurisdiction are developed by the building permit or housing unit method. These small area estimates are then adjusted so that their sum equals the independently derived total control. This process limits the tendency of the housing unit method to overstate population. In effect, the housing unit method serves to establish the geographic distribution of the region's total population which is estimated using different methods and data bases. TMTPnnnrTTrw The Atlanta Regional Commission (ARC) and its predecessor planning agencies have prepared small area population and housing estimates annually since During this time the jurisdiction of the regional planning agency has expanded from two to seven of the 15 Atlanta metropolitan counties and currently includes a total of 53 local governments and 243 census tracts for which separate estimates are prepared. The agency has relied upon the housing unit method to develop the small area estimates because building permits are the only data which are available for all local governments covered by the estimates program and which can be allocated, by address, to the jurisdictional parts of each census tract. In order to offset the tendency of the housing unit method to overstate population, the agency has also controlled the results of the housing unit method to an independently derived estimate of the total regional population

2 This paper describes briefly the ratio method and the multiple regression technique used to develop the control estimate of total regional population. The paper also presents in more detail the housing unit method by which the small area estimates are derived. The housing unit methodology reflects a number of refinements which have been implemented during the past decade to improve the accuracy of the small area estimates. In each case, specific reference is made to ARC's most recent estimates as of April 1, REGIONAL CONTROL TOTAL The population estimate for the seven-county Atlanta Region is derived from two independent methods: the ratio method and multiple linear regression analysis. Results of the two methods are averaged to produce the regional control total for the component parts of the region. Separate estimates for each census tract and jurisdiction are developed by the building permit or housing unit method. These small area estimates are then adjusted so that their sum equals the regional control total. Ratio Method The ratio method is one in which an estimate of the population of a given locality is derived from estimates of the population of a larger geographic area of which it is a part. For this set of estimates, it is necessary to determine how much of the United States population resides in the seven-county Atlanta Region. The major assumption is that the seven-'county area has continued to increase its share of the U.S. population in the same amount as it did from 1960 to According to U.S. Census data, the Atlanta Region's population was percent of the national population in 1960, and in The average increase per year in the region's share of U.S. population in 1978 is percent ( plus eight times ). The April 1, 1978, estimate of the U.S. population is 217,599,000 (4). Using the ratio method then produces a regional population total of 1,751,000 ( percent times 217,599,000, rounded to the nearest thousand). Regression Method In 1976 ARC implemented multiple regression as an additional method of estimating regional population. This refinement was made, in part, because the ratio method was producing population estimates considerably higher than those prepared by the Census Bureau and the Georgia Office of Planning and Budget (OPB), the State agency which participates in the Census Bureau's Federal-State Cooperative Program Moreover, multiple regression permits the flexible use of additional, current variables which are symptomatic of population change

3 ARC first investigated the possibility of using the same data series and regression tehcnique as OPB. However, study of the data used by OPB to estimate population (births, deaths, auto registrations, and school enrollment) revealed some anomalies in auto registrations for Fulton County which is the region's most populous county. In addition, Fulton County has the most extensive bus service in the state, a substantial low-income population, and a high proportion of government vehicles which are required to register only at the time of purchase and every five years afterafter. For these reasons, ARC seriously questions the use of auto registrations as a predictor variable of population for the Atlanta Region. As an alternative, a seven-county total housing data series is used by ARC instead of automobile registrations. This series is part of ARC's annual population and housing data from 1960 through In addition, ARC used OPB's data series on births, deaths, and school enrollment, grades 1-8 plus special education. For the 1978 population estimate, ARC used the REGRESSION subprogram of the Statistical Package for the Social Sciences (SPSS) t.o run a forward (stepwise) inclusion multiple regression (1). This technique enters all predictor variables, one by one, in the order of the respective contribution of each variable to explained variance. The equation developed from the regression estimates or predicts the dependent variable population from the independent variables housing units, births, deaths, and school enrollment plus a constant term. The 1978 equation is: 1978 Population = -26, HU B (1) D SE» 1,711,423 Where: HU = Atlanta Region Housing Units 4/1/78 (655,770) B = Atlanta Region Resident Births 1/1/78-12/31/78 (Estimated 25,700) D = Atlanta Region Resident Deaths 1/1/78-12/31/78 (Estimated 11,200) SE = Atlanta Region Fall 1977 School Enrollment, Public Private Grades 1-8 plus Special Education (Estimated 241,462) and Standard Error = 9220 The value of the multiple R squared of all four variables,.99855, reflects the amount of variance accounted for by the independent variables. The associated F values, which follow, describe the relative contribution of each variable: housing units, ; births, ; school enrollment, 3.669; and deaths, Obviously, housing units is the main predictor

4 HOUSING UNIT METHOD The housing unit method involves the use of information from residential building and demolition permits. Each housing unit which has been authorized for construction, demolition, or conversion is allocated, by street address, to the jurisdictional part of the census tract in which it is located. Following a series of field and adjustment steps, this information is used to update the housing inventory determined by the 1970 Census. The various steps involved in data collection and classification for the building permit method are outlined briefly below: 1. Investigation of possible annexations or other changes in municipal boundaries. 2. Recording of information from residential construction and demolition permits and coding of units permitted by census tract, race, and structure type. 3. Survey of institutional and other non-institutional population. 4. Field survey to determine racial transition of housing units. 5. Survey of resident mobile homes. 6. Field survey to determine status of construction and occupancy of apartments permitted. 7. Determination of necessary adjustments to the housing inventory and other information. The new housing unit figures developed are then used to estimate current population for census tracts and municipalities. In order to convert housing units to population estimates, several major assumptions are necessary: 1. Assumptions about the time lapse between date of permit issuance and date of actual completion or demolition. 2. Assumptions about the number of persons per occupied housing unit. 3. Assumptions about the number of vacant housing units. 4. Assumptions about the number of persons living in group quarters. Time Lapse A period of three months is allowed from the date a permit is issued to the time the housing unit is assumed (1) complete and ready for occupancy, or (2) demolished. Multi-family units, which ordinarily take much longer for construction, are not included in the inventory until the actual date of completion is determined by field survey. Household Size A number of different person per household rates are used in estimating population depending upon a combination of the following

5 factors^ (1) whether the unit was built pre-1970 or post-1970, (2) the race of the family occupying the unit, (3) the structure type of the unit, and (4) the area or census tract in which the housing unit is located. There are two reasons for using several different household size factors. First, an examination of census data indicated that population per household varies both by race and by type of structure. Moreover, the data showed that these factors tend to be larger in suburban areas than in the older section of the central city. Second, in some areas, depending upon construction trends, the types of housing constructed since 1970 are often quite different from those which existed in For example, in a single census tract, all post-1970 housing might be apartments while pre-1970 housing might have been predominately single family. Presumably, in such situations, the pre-1970 and post-1970 household sizes would also differ population per household rates (by race) which were computed for each census tract from 1970 Census data are applied to occupied housing units built before For census tracts which have undergone substantial racial transition since 1970, alternate rates are selected from tracts with similar population characteristics. A different set of household sizes is applied to occupied housing units built after 1970 depending upon the race and type of structure, and the area in which the units are located. The structure types delineated are single-family, duplex, multi-family with three to 49 units per structure, multi-family with 50 or more units per structure, and mobile homes. The areas determined to differ per structure with respect to household size (based on 1970 Census data) are the area inside the 1950 Atlanta city limits, the balance of Atlanta, DeKalb County, outside Atlanta, Fulton County, outside Atlanta, Clayton County, Cobb County, Douglas County, Rockdale County, and Gwinnett County. TABLE 1 AVERAGE HOUSEHOLD SIZES. UNITED STATES AND 1976 Total White and c&fier Races All SMSAs All SMSAs All SMSAs Ratio Ratio = 1976/1970 Source: (3;5). A special adjustment to all household size factors has been made to reflect the general decline between 1970 and 1976 in household sizes throughout the United States. This adjustment was accomplished by applying to the ratio of the U.S to 1970 average household size for all SMSAs by race (white 0.913, black 0.888)

6 Vacant Housing Unit The assumed vacancy rate for pre-1970 housing units in a particular census tract is based on the tract's 1970 ratio of (1) vacant units available for rent or sale to (2) total housing units. When a census tract's 1970 vacancy rate appears unusually high or low for the current housing situation, an alternative rate is selected from another census tract with comparable housing types. For housing units built after 1970, the assumed vacancy rate factors are varied by type of structure for each of two ages of housing units those built during the current year and those built during previous years of the seventies. The specific rates used in developing the April 1, 1978, estimates are presented in Table 2 below (6). The same rates are used for all census tracts in the region and are based upon a special tabulation of 1970 Census data. TABLE 2 VACANCY RATE FACTORS APPLIED TO HOUSING UNITS APRIL 1, 1977 to APRIL 1, 1978 BUILT Single-Family Units 26.44% Duplex Units 9.36% VACANCY RATE FACTORS APPLIED TO HOUSING UNITS APRRIL 1, 1970 to APRIL 1, 1977 BUILT Single-Family Units 1.16% Duplex Units 4.17% Multi-Family Units 3-49 Units per structure 5.68% 50 plus Units Per Structure 6.27% Separate rates are used for single-family and duplex housing built during the current year because the special tabulation of 1970 Census data revealed that new housing units have considerably higher vacancy rates than do older housing units. The same is true of multi-family housing units; however, the vacancy rates of new apartment units are determined by field survey. As a result, the actual number of vacancies is known and no assumed rates are necessary. Similarly, no rates are shown for mobile homes because only those occupied by permanent residents are counted in the housing inventory. Through field surveys, the current vacancy rate is also known for most apartment units built from 1970 to In these cases, the actual rates rather than the assumed rates are used to compute vacancies

7 Group Quarters Population When possible, exact information is obtained on the number of persons living in group quarters as in the case of college domitory residents and inmates of institutions. For census tracts where such information can not be obtained, the assumption is made that the group quarters population has remained at the level reported in the 1970 Census. FINAL ADJUSTMENTS Group quarters population is considered to be known population and, thus, is excluded from the adjustment of small area estimates to the regional control total. The household population only is adjusted down by small area so that the sum of the small area estimates is equal to the regional control total excluding group quarters population. For the April 1, 1978, estimates, selected small municipalities were excluded from this adjustment if the 1978 population totaled less than 25,000 and if housing units built from 1970 to 1978 accounted for less than one-fourth of the city's total 1978 ousing inventory. The estimates by census tract and jurisdiction have been published by ARC in a series of reports titled Population & Housing (2). These reports also include more detailed information on the precise factors used to convert the updated housing inventory to current population. REFERENCES (1) Nie, Norman H., et al, Statistical Package for the Social Sciences, Second Edition, McGraw-Hill Book Company, (2) Population & Housing, Atlanta REgional Commission, 1978 and earlier years. (3) U.S. Census Bureau, Current Population Series P-20, No.218, March (4) U.S. Census Bureau, Current Population Series P-25, No.724, (5) U.S. Census Bureau, Current Population Series P-60, No.104, March (6) U.S. Census Bureau, unpublished special tabulations of 1970 Census of Housing data,