SUBCOUNTY POPULATION ESTIMATES METHODOLOGY Introduction. The Census Bureau calculates subcounty estimates using a housing unit method in which the change in the number of housing units at the subcounty level is used to distribute the county population to subcounty areas. Table 1 shows the derivation of a July 1 population estimate for a typical local government in a typical estimate year. The process may not be applied exactly as described to an individual local government in a specific estimate year. Components of Table 1. Housing Units (Line 1)-- Total housing units from the 1990 Census. Estimated Residential Construction (Line 2)-- Building permits are compiled from internal data files developed by Manufacturing and Construction Statistics Division. These files include imputed permits where a local jurisdiction did not report permit issuance for the entire year. Housing growth calculated from building permits employs a census region-specific lag time (weighted average by number of units in structure) between the issuance of permits and completion of construction. The lag times by census region (in number of months) are: U.S. average 7.5 Northeast Region 8.3 Midwest Region 7.5 South Region 7.2 West Region 8.7 Two percent of all building permits never result in the actual construction of a housing unit (as derived from U.S. Bureau of the Census Current Construction Reports, Series C20-9103 and Series C22-9107). Therefore, a factor of .98 is used to estimate completed new units. Many jurisdictions provide only annual data to the Building Permit Survey. These reporting practices have an impact on the ability of the estimation method to measure seasonal fluctuations in the growth of housing stock. New residential construction was calculated in the following manner for the July 1, 1998 estimates: NC98 = [[(BP89 + BP90 + BP91 + ... BP98] * .98] + NPC98 Where: NC98 = Estimate of new residential construction for the period: April 1, 1990 to July 1, 1998 BP89 = Residential building permits issued in 1989 that would result in the completion of a housing unit after April 1, 1990 and prior to the date of the estimate. BP90 - BP98 = Residential building permits issued in 1990-1998 that would result in the completion of a housing unit after April 1, 1990 and prior to the date of the estimate. NPC98 = Estimate of new residential construction occurring in non-permit issuing areas for the period: April 1, 1990 to July 1, 1998 Note: Estimates of nonpermitted residential construction (NPC98) are calculated using the following steps: At the county level 1. Calculate the number of 1990 county housing units located in non-permit issuing jurisdictions within the county by subtracting the number of units located in permit issuing jurisdictions from the total units 2. Sum the housing units located in non-permit issuing jurisdictions to the national level. 3. Calculate the county's share of units located in jurisdictions not issuing permits by dividing the county's units by the nation's units -- (1)/(2). 4. Multiply the total number of units reported in the Survey of Construction as constructed without building permits by the county's share of units located in jurisdictions not issuing building permits. At the subcounty level 5. If a subcounty area was represented by a permit issuing jurisdiction, the number of nonpermitted units constructed is set to zero. 6. If a subcounty area was not represented by a permit issuing jurisdiction, calculate the subcounty area's share of the county housing units located in non- permit issuing jurisdictions. 7. Apply the proportion in calculated in step 6 to the county estimate of nonpermitted construction. Estimated New Mobile Home Placements (Line 3)-- Tabulations of mobile homes shipped to states are consistent with Series C-40 reports. Ninety-eight percent of all mobile homes shipped to states are used for residential purposes. Internal studies have shown that on average a two month lag time exists between the time when a mobile home is shipped and subsequently put in place for residential use (Schwanz, 1986). These factors result in the development of the following formula. NM98 = [(MH90*(11/12)) + MH91 +...+ MH97 + (MH98*(4/12))] * .98 Where: NM98 = Estimated new residential mobile home placements since April 1, 1990 MH90-98 = Mobile homes shipped to states in 1990 - 1998 State mobile home information is distributed to counties based upon a county's proportion of the state's mobile homes as of the 1990 census. Estimated Housing Loss (Line 4)-- Demolition permits were compiled from internal data files developed by Manufacturing and Construction Statistics Division (MCSD). These files include imputed permits where a permit issuing jurisdiction did not report permit issuance for the entire year. No lag time is assumed for demolition permits. HL98 = (.75 * DP90) + DP91 +...DP94 + (.5 * DP95) + DL98 Where: HL98 = Estimate of housing loss for the period: April 1, 1990 to July 1, 1993 DP90-95 = Residential demolition permits issued in 1990-1995 DL98 = Estimated housing unit loss not measured by demolition permits for the period, April 1, 1990 to July 1, 1998 Note: MCSD stopped collecting data on demolition permits in 1995. After 1995, all data on housing unit loss is estimated. Estimates of nonpermitted demolitions (DL98) are calculated using the following steps: 1. Calculate the county's share of structures in region at risk for demolition using data on the county's share of the following types of structure: 1. Mobile homes and other 2. Older units (pre-1939 construction) 3. Vacant for Seasonal and Recreational Use 4. Boarded up Data from the Components of Inventory Change (CINCH) study show these units to be at a greater risk to loss. 2. Apply the proportion calculated in (1) to CINCH regional loss estimates for uninhabitable units. 3. Apply the proportion calculated in (1) to CINCH regional loss estimates for demolitions and disasters. 4. If specific housing unit loss data due to disaster are known, override step 3. 5. For each year since 1990, rake the difference between the estimated regional total of units lost to disaster and demolitions and the CINCH regional loss data to disaster and demolitions, to all subcounty areas within the region. 6. For each subcounty area, subtract the number of housing units lost to demolitions and disasters as reported on demolition permits from the estimate of housing unit loss to demolition and disaster not covered by demolition permits. If the difference is zero or less, all loss due to demolitions and disaster is covered by demolition permits. 7. Total housing unit loss, not covered by demolition permits, for any subcounty area is the sum of the estimate of units becoming uninhabitable from (2) and the estimate of loss due to demolition and disaster not covered by demolition permits from (6). Note: In essence, this process operates by applying a crude death rate to the housing inventory to develop a baseline estimate of housing unit loss. This crude death rate is refined by the type of housing stock contained within each subcounty area. After application of the crude death rate, if real data exists (demolition permits) for the subcounty area, that data is used as a sub-component of loss. The sum of permitted and non-permitted housing loss exceeds the baseline estimate of housing unit loss only in cases where permitted demolitions, or known disaster losses are greater than the estimate developed by the crude death rate. Housing Units in Estimate Year (Line 5)-- Housing unit estimates for each area were developed by the component model described below. The July 1, 1996 estimates are used here as an example. HU98 = HU90 + (NC98 + NM98) - HL98 Where: HU98 = Estimated 1996 housing units HU90 = 1990 census housing units NC98 = Estimated residential construction, April 1, 1990 to July 1, 1998 NM98 = Estimated new residential mobile home placements, April 1, 1990 to July 1, 1998 HL98 = Estimated residential housing loss, April 1, 1990 to July 1, 1998 1990 Occupancy Rate (Line 6)-- The occupancy rate is calculated by dividing the number of occupied units in the locality by the total number of units as reported in the 1990 Census. 1990 Persons per Household (Line 7)-- The number of persons per household is obtained by dividing the household population as reported in the 1990 Census by the number of occupied housing units in 1990. Uncontrolled Household Population (Line 8)-- The uncontrolled household population is obtained by multiplying the number of housing units in the estimate year by the occupancy rate and the persons per household. Uncontrolled County Household Population Estimate (Line 9)-- The uncontrolled county population is the sum of the uncontrolled subcounty area household population estimates within the county. Published County Estimate (Line 10)-- The published county population estimate as calculated by the tax return method for the current estimate year. County Rake Factor (Line 11)-- The published county population estimate divided by the uncontrolled county population estimate. Controlled Household Population Estimate (Line 12)-- The uncontrolled household population estimate multiplied by the county rake factor. Group Quarters Population (Line 13)-- This component is primarily a combination of military personnel living in barracks, college students living in dormitories and persons residing in institutions. Inmates of correctional and juvenile facilities and persons in health care facilities and Job Corps Centers are also included in this category. Final Population Estimate (Line 14)-- The sum of the Controlled household population estimate and the group quarters population estimate. TABLE 1. Derivation of a Current Year Population Estimate for a Hypothetical Local Government: Value Derivation or Source ------------------------------------------------------------------------ 1. Housing Units in 1990 ....... 14,240 From Decennial Census 2. Estimated Residential Construction: 4/1/90 to Estimate Year............... 2,010 See text for detailed source 3. Estimated New Mobile Home Placements: 4/1/90 to Estimate Year............... 88 See text for detailed source 4. Estimated Housing Unit Loss 4/1/90 to Estimate Year .... 28 See text for detailed source 5. Housing Units in Estimate Year ....................... 16,310 (5) = (1) + (2) + (3) - (4) 6. 1990 Occupancy Rate ........... .978 From Decennial Census 7. 1990 Persons per Household... 2.89 From Decennial Census 8. Uncontrolled Household Population Estimate......... 46,099 (8) = (5) * (6) * (7) 9. Uncontrolled County Household Population Estimate ........ 759,159 See text for detailed source 10. Published County Estimate... 744,415 See text for detailed source 11. County Rake Factor.......... .98057 (11) = (10)/(9) 12. Controlled Household Population Estimate ....... 45,203 (12) = (8)*(11) 13. Group Quarters Population Estimate................... 1,685 See text for detailed source 14. Final estimate.............. 46,888 (14) = (12) + (13) Prepared by: Analyst Date: Any Date SOURCE: Population Estimates Branch U. S. Bureau of the Census