Prince William County Quarterly Population Estimate Methods October 2016 Department of Information Technology Geographic Information Systems (GIS) Division
WHY THE ESTIMATE IS HIGHER From Quarter 3 in 2016 going forward, the Prince William County quarterly population, housing unit, and household estimates will be performed based on the following updated data: Premise address data (all structure addresses in the county) maintained by the Department of Technology s GIS Division. U.S. Census housing occupancy rate (OCCRATE) data, broken down by housing type (SFD: Single-Family Detached, SFA: Single-Family Attached, and MFA: Multi-Family Attached). U.S. Census average household size (AVHHS) data, broken down by housing type (SFD: Single-Family Detached, SFA: Single-Family Attached, and MFA: Multi-Family Attached).
OLD & NEW ESTIMATES The population estimate DIFFERS DRAMATICALLY FROM PREVIOUS ESTIMATES due to the updates. However, accuracy and currency are most important when making such estimates. The table below illustrates the differences: Q2 2016 (June 30 th ) Q3 2016 (Sept. 30 th ) Population 434,183 453,049 Housing Units 148,341 154,934 Households 130,785 148,869 Households were reported based on 2010 Census data, not customized estimates PWC Population Forecasts (COG Round 9.0): 2020: 464,613 2030: 519,084 2040: 556,904 Notes: Q2 Quarter 2 Q3 Quarter 3
NEW OCCUPANCY RATES The housing occupancy rates used to calculate the population and household estimates were updated to reflect current data, which YIELDED HIGHER VALUES. Two customized estimates were combined to reach the final estimate: PUMS 2014 data and Census Blocks 2010. Used for Q2 2016 (June 30 th ) Used for Q3 2016 (Sept. 30 th ) Census PUMS Data 2014 5YR Census Block Est: 2010 Census Adjusted to: ACS 2010-2014 5-YR County-Wide Mean (95.56 %) Single-Family Detached: 93.380 % 97.394 % 97.610 % 96.960 % 97.178 % Single-Family Attached: 96.435 % 95.677 % 96.050 % 95.091 % 95.305 % Multi-Family Attached: 91.000 % 93.333 % 93.109 % 93.343 % 93.553 % Mean: 93.605 % 95.468 % 95.590 % 95.131 % 95.345 % Notes: Q2 Quarter 2 PUMS Public Use Microdata Sample ACS American Community Survey mean
NEW AVERAGE HOUSEHOLD SIZES The average household sizes used to calculate the population and household estimates were updated to reflect current data, which yielded SLIGHTLY LOWER VALUES. Two customized estimates were combined to reach the final estimate: PUMS 2014 data and Census Blocks 2010. Used for Q2 2016 (June 30 th ) Used for Q3 2016 (Sept. 30 th ) Census PUMS Data 2014 5YR Census Block Est: 2010 Census Adjusted to: ACS 2010-2014 5-YR County-Wide Mean (3.150) Single-Family Detached: 3.317 3.350 3.237 3.319 3.464 Single-Family Attached: 3.037 2.956 2.708 3.071 3.205 Multi-Family Attached: 2.240 2.228 2.054 2.302 2.402 Mean: 2.865 2.845 2.666 2.897 3.024 Notes: Q2 Quarter 2 PUMS Public Use Microdata Sample ACS American Community Survey mean
WHERE THE ESTIMATE FITS 650,000 2016 Q2: 434,183 Orange Line: Logarithmic Growth Q3 Blue Line: Weldon Cooper Center Projections Gray Line: Linear Growth Model Other Estimates: -Weldon Cooper Center: 443,463 (2015) -PWC Economic Development (Applied Geographic Solutions, 2016): 453,761 (2016) U.S. Census Bureau; Metropolitan Washington Council of Governments; Prince William County GIS Division CHART GENERATED: OCTOBER 2016
OLD & NEW POPULATION ESTIMATES: TIMELINE Quarter 4 (Q4) Q4 Housing Permits Continue New Estimate 2016 Quarter 3 (Q3) -New Baseline Dataset of Residential Addresses (GIS) -New Average Household Size & Occupancy Estimates Q3 Housing Permits New Population Estimate Quarter 2 (Q2) Q2 Housing Permits New Population Estimate Quarter 1 (Q1) Q1 Housing Permits New Population Estimate Quarter 4 (Q4) Q4 Housing Permits New Population Estimate 2011 Quarter 3 (Q3) Quarter 2 (Q2) Q3 Housing Permits Q2 Housing Permits New Population Estimate New Population Estimate Quarter 1 (Q1) Q1 Housing Permits New Population Estimate DECENNIAL CENSUS (2010, Summary File 1, 100% Data)
VISUALIZATION OF HOUSING UNIT TYPES IN PRINCE WILLIAM COUNTY Occupied residential premise addresses by housing unit type: Notes: Symbols on the map are exaggerated to emphasize the distribution of housing unit types. This map does not communicate population density.
METHODOLOGY OVERVIEW GIS Premise Address Database (updated quarterly with new housing permits listed on EnerGov) New Housing Unit Occupancy Rate Estimate OCCRATE AVHHS New Average Household Size Estimate Summarized by housing unit type Multi-Family: Apartments & Multi-family attached Single-Family Attached Single-Family Detached includes Mobile Home Parks Households calculated based on occupancy rates (housing units occupancy rate) Population calculated based on households & average household sizes (households average household size) Group Quarters Population Total Population with Group Quarters
METHODOLOGY OVERVIEW An attempt to summarize the new estimate process using expressions (Note: conventional symbols are not used in the expression language. Read variable definitions first) a Single-family detached unit (SFD), b Single-family attached unit (SFA), c Multi-family attached unit (MFA) UT abc Housing unit type totals from premise address database SFD (a), SFA (b), MFA (c) PUMa xy b xy c xy Census PWC PUMAs 2010-2014 average household size estimate (x) & occupancy estimate (y) CENa xy b xy c xy Census 2010 SF1 tabulation block household size estimate (x) & occupancy estimate (y) ACSa xy b xy c xy Census 2010-2014 ACS household size estimate (x) & occupancy estimate (y) for entire county PHU abc A given block s percent of housing unit type (a, b, or c) sample total PE Population Estimate GQE Group quarters population estimate for entire county M Mean Summation PCESTa xy b xy c xy M of PUMESTa xy b xy c xy & CENESTa xy b xy c xy PUMESTa xy b xy c xy (PUMa xy b xy c xy PHU abc ) CENESTa xy b xy c xy M of (CENa xy b xy c xy PHU) & (CENa xy b xy c xy PHU) adjusted to ACSa xy b xy c xy PE ((UT abc PCESTa x b x c x ) (PCESTa y b y c y )) + GQE
CENSUS TABULATION BLOCKS: 2010 Data from the 2010 Census at the tabulation block level (smallest unit of analysis) played a role in calculating the new occupancy rates and average household sizes to yield a new population estimate
DETAILED METHODOLOGY Documenting the calculations of the new housing occupancy rates and average household sizes: 1. Average Household Size, Census Tabulation Block Calculations using Census 2010 data, ADJUSTED to the 2010-2014 American Community Survey (ACS) county-wide average household size value 2. Housing Occupancy Rates, Census Tabulation Block Calculations using Census 2010 data, ADJUSTED to the 2010-2014 American Community Survey (ACS) county-wide occupancy rate value 3. Average Household Size AND Housing Occupancy Rate estimates using 2010-2014 5-Year Public Use Microdata Sample (PUMS) estimates.
AVERAGE HOUSEHOLD SIZE: CENSUS BLOCK ESTIMATE AVHHS of Census Block with 100% SFD AVHHS of Census Block with 100% SFA AVHHS of Census Block with 100% MFA Block s Total Units / Total SFD HU in PWC that are 100% of Census Block Total Block s Total Units / Total SFA HU in PWC that are 100% of Census Block Total Block s Total Units / Total MFA HU in PWC that are 100% of Census Block Total Census Blocks in PWC with Housing Units (3,704) that contain: 100% SFD units: 1,728 (59,540 units) 100% SFA units: 377 (13,717 units) 100% MFA units: 144 (10,853 units) SFD AVHHS Weight SFA AVHHS Weight MFA AVHHS Weight Example: Weighted AVHHS average for SFA units in PWC Census Block 564 : AVHHS (2.63) (16 HU / 13,717 SFA in PWC) 0.00307 Census Block 1842 : AVHHS (4.14) (143 HU / 13,717 SFA in PWC) 0.03930 Weighted Mean AVHHS: SFD: 3.319 SFA: 3.071 MFA: 2.302 Sum of weights
AVERAGE HOUSEHOLD SIZE: CENSUS BLOCK ESTIMATE ADJUSTED to 2010-2014 Census ACS Overall AVHHS for PWC AVHHS of Census Block with 100% SFD AVHHS of Census Block with 100% SFA AVHHS of Census Block with 100% MFA Block s Total Units / Total SFD HU in PWC that are 100% of Census Block Total Block s Total Units / Total SFA HU in PWC that are 100% of Census Block Total Block s Total Units / Total MFA HU in PWC that are 100% of Census Block Total Census Blocks in PWC with Housing Units (3,704) that contain: 100% SFD units: 1,728 (59,540 units) 100% SFA units: 377 (13,717 units) 100% MFA units: 144 (10,853 units) SFD AVHHS Weight SFA AVHHS Weight MFA AVHHS Weight 1.0873 1.0873 1.0873 Example: Weighted AVHHS average for SFA units in PWC Census Block 564 : AVHHS (2.63) (16 HU / 13,717 SFA in PWC) 0.00307 1.0873 0.00334 Census Block 1842 : AVHHS (4.14) (143 HU / 13,717 SFA in PWC) 0.03930 1.0873 0.042733 Weighted Mean AVHHS: SFD: 3.319 SFA: 3.071 MFA: 2.302 Mean 2.897 Census ACS 2010-2014 5YR AVHHS for PWC: 3.150 Mean Difference: 0.253: ACS is 8.73% higher Adjusted MEAN AVHHS: SFD: 3.609 SFA: 3.339 MFA: 2.503
AVERAGE HOUSEHOLD SIZE: CENSUS BLOCK ESTIMATE MEAN Weighted Mean AVHHS: SFD: 3.319 SFA: 3.071 MFA: 2.302 Adjusted Weighted Mean AVHHS: SFD: 3.609 SFA: 3.339 MFA: 2.503 Census Block Final Estimate: SFD: 3.464 SFA: 3.205 MFA: 2.402
HOUSING OCCUPANCY RATES: CENSUS BLOCK ESTIMATE OCCRATE of Census Block with 100% SFD OCCRATE of Census Block with 100% SFA OCCRATE of Census Block with 100% MFA Block s Total Units / Total SFD HU in PWC that are 100% of Census Block Total Block s Total Units / Total SFA HU in PWC that are 100% of Census Block Total Block s Total Units / Total MFA HU in PWC that are 100% of Census Block Total Census Blocks in PWC with Housing Units (3,704) that contain: 100% SFD units: 1,728 (59,540 units) 100% SFA units: 377 (13,717 units) 100% MFA units: 144 (10,853 units) SFD OCCRATE Weight SFA OCCRATE Weight MFA OCCRATE Weight Example: Weighted OCCRATE average for SFA units in PWC Census Block 564 : OCCRATE (96.118) (30 HU / 13,717 SFA in PWC) 0.11983 Census Block 1842 : OCCRATE (93.455) (275 HU / 13,717 SFA in PWC) 1.91118 Weighted Mean AVHHS: SFD: 96.960 SFA: 95.091 MFA: 93.343 Sum of weights
HOUSING OCCUPANCY RATES: CENSUS BLOCK ESTIMATE ADJUSTED to 2010-2014 Census ACS Overall OCCRATE for PWC OCCRATE of Census Block with 100% SFD OCCRATE of Census Block with 100% SFA OCCRATE of Census Block with 100% MFA Block s Total Units / Total SFD HU in PWC that are 100% of Census Block Total Block s Total Units / Total SFA HU in PWC that are 100% of Census Block Total Block s Total Units / Total MFA HU in PWC that are 100% of Census Block Total Example: Weighted OCCRATE average for SFA units in PWC Census Blocks in PWC with Housing Units (3,704) that contain: 100% SFD units: 1,728 (59,540 units) 100% SFA units: 377 (13,717 units) 100% MFA units: 144 (10,853 units) SFD OCCRATE Weight SFA OCCRATE Weight MFA OCCRATE Weight 1.0045 Census Block 564 : OCCRATE (96.118) (30 HU / 13,717 SFA in PWC) 0.11983 1.0045 0.12037 Census Block 1842 : OCCRATE (93.455) (257 HU / 13,717 SFA in PWC) 1.91118 1.0045 1.9198 1.0045 1.0045 Weighted Mean OCCRATE: SFD: 96.960 SFA: 95.091 MFA: 93.343 Mean 95.131 Census ACS 2010-2014 5YR OCCRATE for PWC: 95.557 Mean Difference: 0.425: ACS is.45% higher Adjusted MEAN OCCRATE: SFD: 97.396 SFA: 95.519 MFA: 93.553
HOUSING OCCUPANCY RATES: CENSUS BLOCK ESTIMATE MEAN Weighted Mean OCCRATE: SFD: 96.960 SFA: 95.091 MFA: 93.343 Adjusted Weighted Mean OCCRATE: SFD: 97.396 SFA: 95.519 MFA: 93.553 Census Block Final Estimate: SFD: 97.178 SFA: 95.305 MFA: 93.553
ESTIMATES USING PUBLIC USE MICRODATA SAMPLE (PUMS) A similar procedure was implemented for PUMS (U.S. Census). Sample areas in the PUMS are called Public Use Microdata Areas (PUMAs). PUMAs 51244 & PUMA 51246 are located in PWC. 1,677 data samples, each with a count of people. Weight of Housing Unit People Sample Weight Housing Type 01 Mobile home or trailer 02 One-family house detached 03 One-family house attached 04 2 Apartments 05 3-4 Apartments 06 5-9 Apartments 07 10-19 Apartments 08 20-49 Apartments 09 50 or more apartments 10 Boat, RV, van, etc. Weight the weights by housing type: (NP(WGTP/Total of Housing Type). Sum weights by housing type to yield weighted mean of AVHHS by housing type (01,02 SFD; 03 SFA; 04-09 MFA)
PUMAs PUMA 51244 - Prince William County (North & West)--Linton Hall & Gainesville PUMA 51246 - Prince William County (Southeast)--Dale City, Lake Ridge & Woodbridge PUMA 512465 not included in calculations of estimates because it encompasses Manassas & Manassas Park
PUMAs Detail of PUMA 51246 - Prince William County (Southeast)--Dale City, Lake Ridge & Woodbridge
COMBINED: CENSUS BLOCKS & PUMS ESTIMATES OCCRATE MEAN Census Block Estimates: SFD: 97.178 SFA: 95.305 MFA: 93.553 PUMS Estimates: SFD: 97.610 SFA: 96.050 MFA: 93.109 FINAL RATES: SFD: 97.394 SFA: 95.677 MFA: 93.331 AVHHS MEAN Census Block Estimates: SFD: 3.464 SFA: 3.205 MFA: 2.402 PUMS Estimates: SFD: 3.237 SFA: 2.708 MFA: 2.054 FINAL RATES: SFD: 3.350 SFA: 2.956 MFA: 2.228
KEEPING THE ESTIMATES UP-TO-DATE: The county-wide ADJUSTMENT VALUES used for the calculation of housing unit occupancy rates and average household sizes for quarterly population estimates will be updated annually with the American Community Survey and Public Use Microdata 5- Year Estimate releases. The Census Tabulation Block data quarterly population estimates will be updated each decade with the release of decennial census data (Next update: 2021). The premise address database will be updated manually each quarter based on occupancy permits posted on EnerGov over the previous quarter. Occupancy and new construction are updated semi-annually or annually by GIS Division. The GIS Division s extensive database of structure (premise) addresses in the county will be used as the baseline dataset for population estimates from this point forward.