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Centroid - A centroid is the geometric center of a feature. In the UDS Mapper ZIP Codes are mapped to the ZCTA that best fits its location based on these centroids.

County Subdivisions - County Subdivisions are the primary divisions of counties and equivalent entities. They include census county divisions, census subareas, minor civil divisions, and unorganized territories and can be classified as either legal or statistical. Minor civil divisions (MCDs) are the primary governmental or administrative divisions of a county in many states (parishes in Louisiana) and the county equivalents in Puerto Rico and the Island Areas. Census county divisions (CCDs) are areas delineated by the Census Bureau in cooperation with state, tribal, and local officials for statistical purposes. CCDs have no legal function and are not governmental units. CCDs often exist where there are no legally established MCDs. Census subareas are statistical subdivisions of boroughs, city and boroughs, municipalities, and census areas, all of which are statistical equivalent entities for counties in Alaska.

Dominant Health Center - The dominant health center is the health center with the highest number of health center patients in a ZCTA. This is the health center with the highest Share of Patients.

Share of Patients - The percent of total health center patients from the specified ZCTA that were served by the specified health center.

Facility/Point HPSA - A facility HPSA designation, as defined by the Public Health Service Act, is given to FQHCs and RHCs that meet the requirement of providing access to care regardless of ability to pay. FQHC in this definition includes all types of FQHCs: Health Center Program (HCP) grantees, HCP look-alikes, and outpatient health programs/facilities operated by tribal organizations (under the Indian Self-Determination Act) or urban Indian organizations (under the Indian Health Care Improvement Act).

Point HPSAs are only applicable to Alaskan Native and Native American Tribal populations. The Federally Recognized Native American Tribes and Alaskan Natives receive automatic population HPSAs. These HPSAs are represented as a point which is placed at a provider location/ facility within the tribal area.

Federal Poverty Level (FPL) - the threshold of poverty (based on income and family size) determined annually by the U.S. Department of Health and Human Services. For more information on the FPL, please see For information on how poverty is measured in the American Community Survey, please see

Federally Qualified Health Center (FQHC) - A Federally Qualified Health Center (FQHC) is a public or private non-profit health care organization that meets certain criteria under the Medicare and Medicaid Programs (respectively, Sections 1861(aa)(4) and 1905(l)(2)(B) of the Social Security Act.) An organization that meets these criteria is eligible to apply for Health Center Program grant funding from the Health Resources and Services Administration's Bureau of Primary Health Care.

Health Center Program (HCP) Look-Alike - an FQHC that meets all of the eligibility requirements of an organization that receives a Health Center Program grant, but does not receive Health Center Program grant funding.

Health Professional Shortage Area (HPSA) - A HPSA is an urban or rural area, population group, or medical or other public facility which has received federal designation as having a shortage of health care providers. Each year, the Department of Health and Human Services (HHS) evaluates HPSA designations and awards them through state Primary Care Offices (PCO). All federally qualified health centers, their look-alikes and tribal facilities receive automatic HPSA designation, while Rural Health Clinics must request HPSA designation. More information can be found here:

Geographic Information Systems (GIS) - A geographic information system (GIS) integrates hardware, software, and data for capturing, managing, analyzing, and displaying all forms of geographically referenced information. GIS allows visualization, understanding, questioning, and interpretation of data in many ways that reveal relationships, patterns, and trends in the form of maps, globes, reports, and charts.1


Health Center Program (HCP) Grantee - a public or private non-profit health care organization that meets certain criteria under the Medicare and Medicaid Programs (respectively, Sections 1861(aa)(4) and 1905(l)(2)(B) of the Social Security Act and receives funds under the Health Center Program (Section 330 of the Public Health Service Act) (i.e., Community Health Centers, Migrant Health Centers, Healthcare for the Homeless Programs, and Public Housing Primary Care Programs). For more information on Health Center Program grantees see the HRSA website. A detailed explanation of health center program terminology is also available from the HRSA website.

Medically Underserved Area/Population (MUA/P) - A medically underserved area (MUA) may be a whole county or a group of contiguous counties, a group of county or civil divisions or a group of urban census tracts that the Department of Health and Human Services (HHS) has designated as having a shortage of health services for residents. Designations are based on the qualifications outlined in the Index of Medical Underservice (IMU), published in the Federal Register on October 15, 1976.

A medically underserved population (MUP) may include groups of persons who face economic, cultural or linguistic barriers to health care. Designations are also based on the qualifications outlined in the Index of Medical Underservice (IMU), and exceptional MUP designations are based on the provisions of Public Law 99-280, enacted in 1986.

For more information, see the HRSA website:

National Health Service Corps (NHSC) - The NHSC program aids HPSAs in attracting the necessary medical, dental and mental health providers to meet the health care needs of the underserved area. The scholarship and loan repayment programs are awarded to physicians who fulfill an obligation to serve a HPSA with a sufficiently high score for underservice. The Bureau of Clinician Recruitment Services (BCRS) oversees the program for the Health Resources and Services Administration (HRSA). For more information see:

Penetration rate - is a ratio of all patients (from health centers with 11 or more patients in that ZCTA) to a sub-population (such as the total population or low-income population). In other words, for each ZCTA the number of reported health center patients is divided by the number of low-income or total residents.

Note: Because the UDS does not distinguish the income of health center patients at the ZIP Code level, this measure is not perfect, particularly when dividing the number of health center patients by the number of low-income (as not all health center patients are low income/below 200% FPL). But because it is known that approximately 92.2% of health center patients nationally are low income, the basic utility of the calculation in assessing the role of health centers in serving the community is not changed. Such penetration rate measures should be considered the starting point for exploring potential unmet need, not the final answer.

Primary Care Association (PCA) - A PCA is a regional, state, or local organization which works in close concert with, and represents the interests of, nonprofit community clinics and health centers and advocates for the health needs of their distinctive populations and geographic areas, most importantly those who face barriers to care due to poverty, language, or geographic isolation.

Primary Care Office (PCO) - A PCO is a state government entity, often under the purview of the state Department of Health, which works to improve access to care for underserved populations. PCOs work with the various bureaus of the Health Resources and Services Administration (HRSA) to promote the community health center program, establish HPSA designations and find suitable locations for placement of NHSC scholar and loan repayors.

Rural Health Clinic (RHC) - The RHC program strives to be the major provider for primary care services for Medicaid and Medicare patients in rural communities which tend to have health disparities due to geographic isolation and low physician density. RHCs can be public, private, or non-profit entities. All RHCs are eligible for facility HPSA designation and the federal funding that comes from that designation, but due to their patient population, RHCs' main funding sources are enhanced reimbursement rates for providing Medicaid and Medicare services to rural populations. As such, RHCs must be located in underserved rural areas, as designated by HRSA, and must employ midlevel practitioners (i.e. physician assistants or nurse practitioners) alongside physicians as part of a team-based approach to patient care. For more information, see:

Uniform Data System (UDS) - The UDS is the specific data collection and reporting requirements for Health Center Program grantees developed by the Health Resources and Services Administration (HRSA) to track the patient population and effectiveness of the health care services of the Health Center Program. The organizations that receiving funding through one or more of the Health Center Program's four funding mechanisms are: Community Health Centers, Migrant Health Centers, Health Care for the Homeless, and Public Housing Primary Care.1 According to HRSA, "the [UDS] data helps to identify trends over time, enabling HRSA to establish or expand service to targeted populations, and identify effective methods and interventions to improve the health of underserved communities and vulnerable populations.2" Thus, UDS data are a vital component of the community health center program, enabling HRSA to inform the expansion of service to low-income medically underserved areas and populations.



ZIP Code Tabulation Areas (ZCTAs) - ZIP Code Tabulation Areas (ZCTAs) are generalized representations of US Postal Service ZIP Codes. Currently, each ZCTA is built by aggregating Census 2010 blocks, whose addresses use a given ZIP Code, into a ZCTA which gets that ZIP Code assigned as its ZCTA code. While in most instances the ZCTA code equals the ZIP Code for an area, not all ZIP Codes have their own ZCTA. For more information, see: