October 2011



State Data Spotlight:
Washington State County-Level
Population Estimation Model
The Office of Financial Management (OFM) in Washington State has
developed an estimation and projection model that can be used to
predict county-level population estimates for 2010 through 2020.
Using data from the American Community Survey and the 2000 Census,
as well as state-level data from OFM and the state’s Department of Social and Human Services, the model
generates population estimates by demographics (age, gender, race, ethnicity), income by specific
poverty level, and health insurance status.

The model was created through funding by the State Health Access Program (SHAP), which began in
October 2009. SHAP is a grant program offered by the Health Resources and Services Administration
(HRSA) at the Department of Health and Human Services (DHHS). Washington is one of thirteen states
chosen for the program that aims to support the expansion of health coverage to uninsured populations.
For more information about Washington’s SHAP initiatives, see the SHAP Grant Summary on the SHADAC
website at http://www.shadac.org/files/shadac/SHAP_GrantSummary_WA.pdf.

The development of this model was motivated by the need for data to support the required planning and
policy development of the Patient Protection and Affordable Care Act (ACA) implementation. The model
goes beyond OFM’s typical county-level demographic data by providing estimates for populations likely
to be covered by the low-income Medicaid expansion and the healthcare exchange. This sub-state
information is critical for understanding how to best approach key regional issues. For example, by
identifying areas of the state with large numbers of uninsured residents, resources may be better
targeted to meet their demand for medical services when health care coverage becomes available under
the ACA. The county-level projections from the model are also available for years through 2020.


Generating County-Level Estimates
Washington’s county-level model involves three key data streams to calculate projections. The
estimation model utilizes 2009 American Community Survey (ACS) data as a base. The smallest
geographic unit available for one-year estimates using ACS is the Public Use Microdata Area (PUMA),
which encompasses a population size of roughly 100,000. PUMAs are typically groups of counties, or
portions of counties in highly populated areas.

For the model’s initial year county-level information, OFM uses ACS data and its own population
estimates to either aggregate up to or allocate to the county level. For future years, the model utilizes the
OFM population projections.




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                                     There are also two assumptions applied to the model: income
                                     distribution adjustment factors and health insurance distribution.
                                     The income distribution adjustment factors modify the size of
                                     population at specified income levels over time. The health insurance
                                     distribution allocates the population within demographic cells across
                                     coverage types for each year.



  OFM population projections              ACS population estimates of                 OFM county-level
    of key demographics                demographics plus income & health            population projections
                                                  insurance
                                 +                                            +


                                              Apply Assumptions




                    County-level population estimates/projections for 2010 through 2020


The model allows for changes to the income distribution adjustment factors and health insurance
distribution assumptions, which provides the opportunity to evaluate alternative scenarios.

States looking to develop similar models can utilize this model as an example for how to incorporate
state-level demographic data with estimates from a Census survey like the American Community Survey.

Federal Reform Implications
Washington’s projection model has advantages as the state moves towards implementation of federal
health reform. Instead of the previous biennial estimates, the new model is able to generate annual
estimates. OFM utilizes data sources that are currently available, increasing the fiscal sustainability of the
model by negating additional costs for data collection. The projection model can also accommodate
alternative scenarios depending on the state’s economic forecast.


Source
Washington State Non-Competing Continuation Progress Report [H2PHS 16400]. (2011). Attachment 7-
Population Estimation Model Project Summary - OFM Forecasting Division.


About the Data Spotlight Series
State Data Spotlight is a SHADAC series highlighting states’ unique data tools, datasets, and uses of
existing data systems. The series aims to provide information and insight on innovations that can be
applied in other states. States pursuing similar strategies can contact SHADAC for technical assistance
and support in developing these types of innovations.
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