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