The Tennessee State Data Center, along with other similar organizations across the county, are tasked by the U.S. Census Bureau with drafting geographic features called Public Use Microdata Areas (PUMAs)

When updating PUMA boundaries population increases and decreases must be weighed along with a number of other criteria. This is to ensure that the microdata provided for each of Tennessee’s PUMAs are useful to researchers and comply with Census Bureau guidance.

Key PUMA Delineation Criteria and Considerations

To protect the confidentiality of responses released in the Public Use Microdata Samples (PUMS), a key requirement is that each area contain a minimum of 100,000 people. However, there are a number of requirements and considerations which each state must weigh when drafting their PUMAs.

Required PUMA Criteria (U.S. Census Bureau)

• Must not cross state boundaries
• Must exceed 100,000 people and maintain this population throughout the decade
• Must be constructed of counties and/or census tracts
• Portions of counties included in PUMAs which cross a county line must be in excess of 10,000 people

Other Guidance (U.S. Census Bureau)

• Should not contain more than 200,000 people wherever possible
• Should be comprised of a single, geographically contiguous area
• Should be comprised of areas entirely within or outside of 2018 core-based statistical areas (metropolitan and micropolitan statistical areas)
• Should maintain relationships with other geographies including corporate limits and 2010 urban/rural delineations
• Should avoid splitting 2010 Census Bureau urban areas.
• Demographically homogenous

Tennessee Specific Considerations

• Where possible, counties with population in excess of 100,000 people should be preserved as individual PUMAs
• Where feasible, maintain existing PUMAs boundaries to provide comparability between PUMS vintages

In addition to the Census Bureau guidance used to draft these initial 2020 proposals, input was also sought from Tennessee State Data Center Affiliates representing the state’s largest counties including Shelby, Davidson, Knox, Rutherford and Williamson County. These members were consulted regarding the creation of the initial draft proposals in regard to the creation of sub-county PUMAs in these areas.

Draft Proposals

Two alternatives were created for initial consideration by data users in Tennessee. Revision and a final alternative will be selected based on input received.

Alternative 1:  Minimal Revisions

Alternative 1 addresses several required changes but otherwise leaves boundaries from 2010 intact. It places a premium on comparability between 2010 and 2020 areas but does not address other recommended changes.

There are two Tennessee PUMAs delineated in 2010 where the 2020 population had grown above 200,000 people. Surpassing this threshold means that additional 100,000+ person PUMAs can be created. Additionally, several census tract boundaries in Shelby County have shifted, thereby necessitating changes that adapt to the new geography.

White outlines show the boundary of the Alternative 1 proposal. Red dashed lines show 2010 PUMA boundaries. Use the +/- button in the top left hand corner of the map to zoom in and show more detail.

Summary of Alternative 1 Changes

1. Reconfiguration of Shelby County PUMAs to meet and maintain required 100,000 person minimum
   Several PUMAs within the southern part of Shelby County were adjusted to achieve the necessary 100,000 person minimum. Additional area was also added to two PUMA’s on the southern edge of Shelby County to ensure these geographies maintain sufficient population throughout the upcoming decade since both had lost population over the last ten years.
2. Montgomery & Stewart Counties–Clarksville City PUMA
   In 2020, this PUMA’s population had grown to 233,726 people. An area entirely within the City of Clarksville corporate limits, that exceeded 100,000 people, was added to create a new PUMA. The balance of Montgomery and Stewart County become the second PUMA.
3. Williamson County–Franklin & Brentwood Cities PUMA
   The population of this county’s single PUMA in 2020 was 247,726. This created an opportunity to split the county into two PUMA’s. An approximation of the boundaries of Franklin and Brentwood were used to create a new PUMA and the balance of the county became a second area.

Alternative 2:  Incorporate Other Census Bureau Guidance and Maximize Data Availability

PUMA’s boundaries that more closely adhere to the Census Bureau’s 2020 delineation guidance were drafted for Alternative 2. The scenario also attempts to maximize the number of PUMA’s in the state so as to improve data granularity.

White outlines show the boundary of the Alternative 2 proposal. Red dashed lines show 2010 PUMA boundaries. Use the +/- button in the top left hand corner of the map to zoom in and show more detail.

Summary of Alternative 2 Changes

1. Reconfiguration of Shelby County PUMA’s to meet and maintain required 100,000 person minimum
   See notes for Alternative 1.
2. Split existing PUMA’s with 2020 population in excess of 200,000 people
   • Montgomery & Stewart Counties–Clarksville City PUMA was divided into two standalone PUMA’s in Clarksville and Montgomery County. Stewart County was added into an adjacent area
   • Williamson County–Franklin & Brentwood Cities PUMA (See Alternative 1)
3. Maximize interior delineations for all counties with greater than 300,000 people
   There are several counties where additional interior delineations would result in new PUMA’s being created. These include:
   • Nashville/Davidson County (pop. 715,884) – increase from five to six PUMA’s
   • Knox County (pop. 478,971) – increase from three to four PUMA’s by inserting splits that coincide with the county’s planning sectors – stable areas built on census tracts
   • Rutherford County (pop. 341,486) – increase from two to three PUMA’s
4. Create standalone PUMAs for counties with more than 100,000 people
   One additional PUMA was created for Maury County. Bradley County, which exceeds the 100,000 person PUMA minimum, remains in a PUMA connected to Polk County to encompass the entirety of the Cleveland, TN Micropolitan area
   • Maury County (pop. 100,974)
5. Improve alignment with 2018 core-based statistical areas boundaries
   Several changes were made to better align metropolitan and micropolitan areas with PUMA boundaries. Where possible, PUMA’s were created to contain:
   • Counties from one metropolitan statistical area
   • Counties from one or more micropolitan statistical areas
   • Areas entirely outside of a core-based statistical areas

In some unavoidable instances, population or contiguity requirements dictated that PUMA’s containing a portion of a metropolitan statistical area and county designated a micropolitan area be combined into a single PUMA.

Evaluation Metrics

Construction of 2020 PUMA’s alternatives comes with tradeoffs. On one hand, maintaining comparability between decades limits the state’s ability to conform with other criteria reduces the number of areas for which data is provided. On the other, changing boundaries to improve the amount of available data may hinders comparison over time.

To help gauge the differences between alternatives, seven criteria were identified. The criteria include

• Total number of PUMA’s
• Number of PUMA’s preserving 2010 boundaries
• Number of PUMAs containing split Core-based statistical area (portion of multi-county metropolitan statistical area along with other entirety of another micropolitan area)
• Number of PUMA’s containing a portion of a county along with the totality of another
• Median PUMA population
• Minimum and Maximum PUMA size
• Number of counties over 100,000-person counties not in standalone PUMA

The initial alternatives score as follows: