Comparing Decennial Census Counts and 

Local Population and Household Estimates:

A Case Study in Fairfax County, Virginia

Fairfax County

Department of Management and Budget

Economic, Demographic, and Statistical Research (EDSR)

Countywide Data Analytics

Presented by:

Xuemei Han, Principal Data Scientist

xuemei.han@fairfaxcounty.gov

October 24-26, 2023

Hyattsville, MD

U.S. Bureau of Labor Statistics Fei Carnes, Yan Jiang, Michelle Gregory (Fairfax County)

Xingyou Zhang (BLS)
Objectives

Why and how does Fairfax County produce small area 

official estimates and forecasts on housing, population, 

and households?

How does Fairfax County local data compare to the 

Decennial Census?

How can we integrate local and federal-level statistical 

data systems to improve data quality and optimize 

mutual resource utilization?

2
Objectives

Why and how does Fairfax County produce small area 

official estimates and forecasts on housing, population, 

and households?

How does Fairfax County local data compare to the 

Decennial Census?

How can we integrate local and federal-level statistical 

data systems to improve data quality and optimize 

mutual resource utilization?

3
Why local estimates and forecasts?

• Local governments need current, relevant, and accurate data in 

custom areas for planning, community service delivery, and resource 

allocation.

• Fairfax County has leveraged administrative data to estimate and 

forecast demographic data to fill the gap in available federal statistics 

for 40+ years. 

https://www.fairfaxcounty.gov/demographics/reports

Fairfax County uses a GIS-based 

spatial-explicit model – Integrated 

Parcel Lifecycle System (IPLS) to 

produce local estimates and 

forecasts.

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Integrated Parcel Lifecycle System (IPLS)

Collects, links, verifies, and utilizes the most current administrative data from multiple databases.

Produces new data, which is the most accurate for Fairfax County at the parcel level.

Data products are updated annually, with point-in-time data as of Jan 1.

Provides estimates and forecasts (up to 30 years).

Housing Units

Households

Population

Housing Value

Development

Pipeline

Gross Floor Area

Existing

Land Use

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Integrated Parcel Lifecycle System (IPLS)

Data is stored in a spatial database, 

with Parcel Identification Number 

and Spatial Location as primary keys.

Data can be summarized at any 

custom geographies.

Primary tools: Spatial SQL Server 

Database, Esri ArcGIS, SAS/R.

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Dept. Info. Technology

• Parcels & Centroids

• Master Address File

• Parcel History

• Various Geographies

• High-resolution aerial 

photo

• Approved 

o Rezoning

o Housing Units

o Gross Floor Area

• Proposed Rezoning

IPLS

Integrated Parcel 

Lifecycle System 

GIS

Site Plans

Tax

Administration

Planning

Permits & 

Inspections Zoning

• Development Plans

o Proposed

o Approved

• Project Types

• Housing Units

• Building Permits 

Issued

• Building Permit 

Inspections

• Comprehensive Plan

• Planned Land Use 

Layer

• Plan Options (Mixed 

Use)

Demographic 

Data

EDSR

• Avg. Household Size

• Vacancy rates

• Owner

• Renter

• Forecast Assumptions

Real Estate File 

• Characteristics

o Land/Lot

o Res/ Com

• Property Info

• Existing Land Use 

Fairfax Water

• Consumption Report

• Account Status

Census Bureau

• Migration

• Avg. HHLDS

CDC

• Birth Rates

• Survival Rates

USPS

• Vacancy

FCPS

• Enrollment

Commercial

• CoStar, Placer.AI

Additional 

Data

Base Base

30-yr

Assumption

Data

IPLS Data Input

Planning and Land Use System 5-yr 7
Estimate Methodology

Real Estate Tax Assessment & GIS files

• Example determinants: Type of unit, age of the structure, percent 

complete, structure condition, market value, existing land use, 

zoning, aerial photo, and access to utilities.

Housing Units

Households = Occupied Housing Units 

• Owner vacancy: Fairfax water consumption rates

• Renter vacancy: Fairfax County rental housing survey research

• Supplementary vacancy: USPS quarterly data

Households

Population = Households x Household Size + Group Quarter Population

• Average Household Size: U.S. Census Bureau, School Enrollment, 

locational movement from mobile apps

• Group Quarter Population: Fairfax County GQ survey research

Population

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Forecast Methodology

1-5 years

• Residential Development Activities

• Rezoning Pending -> Rezoning Granted -

>Development Plan Submitted ->Development Plan 

Approved -> Building Permit Issued -> Under 

Construction

Short-term 

forecast

6-30 years

• Comprehensive Plan

• Mixed Used Plan Options 

• Parcel Characteristics: age of structure, location, 

steepness, flood plains, etc. 

Long-term 

forecast

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Data Output Example: 

Forecast Population Density and Growth in 30 years

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Forecast Accuracy

5-year projection accuracy: 99.3%

• Evaluation data: 2007-2022

• Average percent error: 0.7% over-forecasted

Short-term 

forecast

10-year projection accuracy: 97.3%

• Evaluation data: 2007-2022

• Average percent error: 2.7% over-forecasted

Long-term 

forecast

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IPLS Output • Demographic Reports • Fairfax County Overview Visualization • MWCOG Regional Forecasting Reports • Demographic Mapper • GIS Open Data Portal • SQL PGIS Database • Ad hoc information and analysis 

services

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Objectives

Why and how does Fairfax County produce small area 

official estimates and forecasts on housing, population, 

and households?

How does Fairfax County local data compare to the 

Decennial Census?

How can we integrate local and federal-level statistical 

data systems to improve data quality and optimize 

mutual resource utilization?

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2020 Decennial 

Census PL 

2020 Fairfax 

County IPLS

Difference 

(Census-IPLS) %Diff

Population 1,150,309 1,171,848 -21,539 -1.8%

Housing Unit 427,149 424,087 3,062 0.7%

Household 411,055 417,464 -6,409 -1.5%

Group Quarter 

Population 11,396 12,215 -819 -6.7%

Average Household Size 2.77 2.78 -0.01 -0.3%

Vacancy Rate 3.8% 1.6% 2.2% 141.5%

Owner Vacancy 0.7% 0.6% 0.1% 20.7%

Rental Vacancy 5.4% 3.9% 1.5% 38.8%

Housing Tenure

Owner-occupied 65.7% 66.6% -0.9% -1.4%

Renter-occupied 34.3% 33.4% 0.9% 2.7%

Comparisons: 

DP1|Profile of General Population and Housing Characteristics

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Comparisons

At the county level • Fairfax County's local data aligns with Decennial 

Census figures for Housing Units, Average 

Household Size, Owner Vacancy, and Housing 

Tenure. 

• Fairfax County’s local estimates for households and 

the overall population, including group quarter 

population, exceed Decennial Census figures.

• Fairfax County's local estimates indicate lower 

rental vacancy rates than the Decennial Census.

At the census tracts and block group level • Geographic variations are observed.

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The COVID-19 pandemic introduced changes in Group Quarter population and housing 

occupancy between January and April 2020.

Differential privacy noise undercounts the population and households in large counties, 

which will affect many other federal statistics beyond Decennial Census counts.

Slight differences in definition can also be attributed to discrepancies, such as in Housing 

Units.

Geographical misplacement of housing units, households, and population onto adjacent 

blocks or tracts. Corrected through the 2020 Census Count Question Resolution operation.

The local model helps to detect and address these discrepancies and 

provides more accurate statistics. 

Why the discrepancies?

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Objectives

Why and how does Fairfax County produce small area 

official estimates and forecasts on housing, population, 

and households?

How does Fairfax County local data compare to the 

Decennial Census?

How can we integrate local and federal-level statistical 

data systems to improve data quality and optimize 

mutual resource utilization?

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What has been done?

Fairfax County has used local population and house data to 

assist the decennial census throughout various stages: LUCA, 

PSAP, Redistricting, CQR, and methodology evaluation to 

provide feedback to the Census Bureau. 

Fairfax County has leveraged federal statistical data products 

to fill local data gaps and facilitate data-driven decisionmaking and evidence-based policymaking. 

Fairfax County has made federal statistics readily accessible to 

the public in user-friendly formats.

https://www.fairfaxcounty.gov/demographics/datavisualization-gallery 

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Our Wishlist

Localities and federal statistical agencies strengthen communication, streamline partnerships for productive 

collaborations, and mutually support optimizing resource utilization.

Federal statistical agencies consider local planning and service needs when designing surveys and programs, 

e.g., the most frequent language spoken at home.

Federal statistical agencies try to have accurate and reliable data for sub-county and smaller geographies for 

localities to consume, e.g., disclosure avoidance implementations.

Federal statistical agencies share customizable toolkits for localities to connect to or regionalize, e.g., Data 

tools, interactive applications, and visualizations.

Federal agencies could provide data standards guidance and quality control procedures for local governments, 

e.g., Statistical Quality Standards.

The mechanisms of the Integrated Parcel Lifecycle System can be transferrable and scalable for use in other 

counties or local governments.

Fairfax County’s administrative data-derived statistics can systematically support the Census Bureau and other 

federal statistical agencies’ programs to reduce the data collection burden and improve data quality. 

https://www.fairfaxcounty.gov/demographics/american-community-survey

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How can we integrate local and 

federal-level statistical data systems to 

➢ reduce data collection burden, 

➢ improve data quality, 

➢ and optimize mutual resource 

utilization?

xuemei.han@fairfaxcounty.gov

https://www.fairfaxcounty.gov/demographics

Where is 

the 

potential? 

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