Circle City Data | Data Documentation
One of the most important sources powering Indianapolis data on Chart Book is the U.S. Census Bureau’s American Community Survey, or ACS. This post explains what the ACS is, how the data is collected, and what you should know when interpreting the numbers you see here.
Before we dig deeper into the ACS, a quick note on finding ACS data in Chart Book. If you want to see ACS data in Chart Book, just search “Census” in the search bar, and the results will show you available datasets.
The ACS is an ongoing nationwide survey conducted by the U.S. Census Bureau. Unlike the decennial census — which counts every person in the country once every ten years — the ACS runs continuously, collecting detailed information about the social, economic, housing, and demographic characteristics of communities across the United States.
What Is the American Community Survey?
The survey produces two main data products: 1-year estimates and 5-year estimates. Chart Book currently uses ACS 1-year estimates, which reflect a single calendar year of data collection. These estimates are more current than the 5-year version, making them well-suited for tracking year-over-year trends. The tradeoff is statistical reliability — 1-year estimates have larger margins of error than 5-year estimates and are only published for geographic areas with populations of 65,000 or more.
That brings us to geography.
Marion County as the Geographic Focus
All ACS data on Chart Book is scoped to Marion County, Indiana. Given the spirit and intent of Circle City Data and following the Unigov consolidation of Indianapolis and Marion County in 1970, county-level data is more than appropriate for our uses here.
When you see a data point on Chart Book, it represents conditions across Marion County as a whole. The ACS does not break data down to the neighborhood level in these estimates. For that level of granularity, the 5-year estimates and Census tract data would be required.
How the ACS Is Conducted
Each month, the Census Bureau mails survey questionnaires to a random sample of addresses across the country. Roughly 3.5 million households receive the ACS annually, selected using a systematic random sampling method from the Census Bureau’s comprehensive inventory of U.S. housing units and group quarters.
Households that don’t respond by mail are followed up via phone and, if necessary, in-person interview by Census Bureau field representatives. This multi-mode approach is designed to maximize response rates and reduce nonresponse bias.
The survey itself asks about a wide range of topics, including income and employment, educational attainment, housing costs and tenure, commute patterns, health insurance coverage, and demographics like age, race, and ancestry. See, for example, the chart below on Educational Attainment for Native and Foreign-Born Populations in Marion County, which is available in Chart Book.
Take note of the “less than high school graduate” population across different groups (the second bar from top). The differences are significant and trigger endless questions. What barriers to education do non-US citizens face? Are foreign-born non-US citizens on a path to get an education?
Participation in the ACS is legally required under Title 13 of the U.S. Code, which also mandates that the Census Bureau keep all individual responses strictly confidential. The data published in products like those used on Chart Book contains no personally identifiable information.
Where to Learn More
The Census Bureau publishes extensive documentation on ACS methodology, including its sample design, weighting procedures, and accuracy measures. If you want to go deeper, the ACS Methodology page is the authoritative starting point.
ACS data available in Chart Book today includes:
- Median Income
- Industry of Employment
- Occupations
- Supplemental Nutrition Assistance Program (SNAP) recipients
- Educational Attainment
- Employment Status
- Vehicles Available per household
- Commuting Characteristics
As Chart Book grows, additional data products may be added.