What Exactly is a Census Tract?

Feb. 12, 2020

Taylor Lampe

The City Health Dashboard offers metrics at both the city and census tract level. Unless you’re a census nerd, you probably don’t think about your city in terms of census tracts. Most of us are much more familiar with our neighborhoods, zip codes, subdivisions, school districts, congressional districts - pretty much any other way of thinking about where we live. So why do we use census tracts on the Dashboard? Let’s dive in!

Athens SidebySide

Left: Athens, GA: Third-Grade Reading Proficiency City-Level Estimate

Right: Athens, GA: Racial/Ethnic Diversity Tract-Level Estimates

Why do census tracts exist?

As early as 1890, the U.S. Census Bureau recognized the need for small geographic boundaries that could show trends in population demographics and housing characteristics over time. Census tracts were the result of this work, created to approximate neighborhoods. Tracts cover the entire country, generally follow recognizable features like roads, rivers, and power lines, and when possible, contain populations with similar characteristics.

Why does the Dashboard use census tracts?

The U.S. Census Bureau is a federal agency that serves as the nation’s leading provider of data about US residents. As part of that, the bureau is the primary authority for statistical geographic boundaries in the United States, determining where cities and tracts begin and end. National public health datasets routinely use census tracts to approximate neighborhoods across the United States. Since census tracts are standardized and commonly used in public health research, this allows Dashboard users to make direct comparisons across cities and datasets.

How can I use census tracts if they don’t align with how our city thinks of neighborhoods?

Most cities are made up of neighborhoods – from Chelsea in New York City to German Village in Columbus, OH. Sometimes census tracts align perfectly with local ideas of neighborhood boundaries, making it easy to understand tract maps on the Dashboard. Other times, though, census tract boundaries do not reconcile with how locals think about their city.

In Providence, RI, for example, there are more census tracts than neighborhoods. While some of the boundaries are similar, generally there’s not a perfect overlap. Still, you can look at the map of census tracts to identify trends across the city, as seen here with physical inactivity. Even if we can’t be certain the exact values of physical inactivity in the city’s neighborhoods, we can see that the southern areas of Providence are less physically active (darker blue) than the northern areas.

Providence SidebySide

Left: Providence, RI Census Tracts

Middle: Providence, RI Neighborhoods

Right: Providence, RI: Physical Inactivity Tract-Level Estimates

We are working on ways to adapt tract-level estimates to get values for custom city neighborhood boundaries. Stay tuned, and email us at [email protected] to get involved.

What do the census tract numbers mean?

Like all census geographies, census tracts have special identifying numbers (also known as FIPS codes). These numbers are six digits and can be presented in various formats, including 0123.40, 012340, or 123.4 (as seen on the Dashboard, with leading and following zeros removed).

The last two digits indicate versions of the census tract if that tract has changed over time. In conjunction with the identifying numbers for states and counties, every tract in the country thus has a unique number.


Image: Portland, ME: Park Access Tract-Level Estimates

Do census tracts ever change?

The Census Bureau works to keep tracts consistent across time to better track population changes. Sometimes, though, it is necessary to split or combine tracts as populations change (see example below of a tract splitting between 2000 and 2010). This typically happens once every 10 years during the big decennial census. The next decennial census will happen this year, starting in April. You may notice a few changes to the map of your city in the next few years!

Boise Tract Split 1.13.20

Image: Boise, ID Tract Splitting, 2000-2010

How big are census tracts?

The population ranges from about 1,500 to 8,000 people per census tract (with an average around 4,000). Depending on how dense the population is, census tracts can be just a few city blocks, like in Chicago, IL, or they can contain a few miles of land, like in Charleston, WV.

Chicago and Charleston

Left: Chicago, IL: Income Inequality Tract-Level Estimates

Right: Charleston, WV: Frequent Mental Distress Tract-Level Estimates

How do census tracts relate to other census geographies?

You may have seen mention of census blocks – these are not the same as census tracts. Blocks are the smallest census geography. Census blocks “nest” within block groups, which nest within tracts. Tracts nest within counties, and counties within states. But, tracts do not fit perfectly inside cities. For the Dashboard, we focus primarily on tracts rather than the census blocks.

census relationships image

Image: Census Geographic Unit Relationships

If tracts don’t fit perfectly within cities, what are we seeing on the site?

A challenge for the Dashboard is that census tracts do not fit perfectly within cities. For some tracts on a city’s border, what you see on the Dashboard are actually tracts that have been visually “cut” to fit within the city boundaries. The values that we present on the site are for the “full” census tract, not only the portion that you are seeing (except for the 500 Cities Project metrics). This means that some tract values may be influenced by populations living outside of the city boundaries.

Fort Wayne Side by Side

Left: Fort Wayne, IN: Lead Exposure Risk Index Tract-Level Estimates

Right: Fort Wayne, IN: City Overlaid on Full Tract Boundaries

How many census tracts are on the Dashboard?

We currently have 32,424 census tracts on the site. Since some tracts are in multiple cities, though, our full list of tracts and their cities contains 33,660 rows of data.

Connect with us to learn more!

If you have questions about census tracts, how we use them on the Dashboard, and more, please reach out to us at [email protected].

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