Data Librarian
Getting Started
Welcome! This guide was designed for the "Census Basics" Workshop at UVA Library, Fall 2019.
These are your learning outcomes:
- Understand the available Census surveys, and their strengths and weaknesses,
- Understand Census geographies,
- Understand where to find easy-to-download data,
- Recognize Standard Error in the ACS and how to compare estimates,
- Recognize advanced download options and other specialized Census products.
Do you still have questions? I invite you to schedule a one-on-one consultation!
Most Common Census Surveys: Decennial Census and American Community Survey (ACS)
The two most commonly used Census surveys are the Decennial Census and the American Community Survey.
The Decennial Census is conducted every 10 years; the most recent one was in 2020. The Census attempts to count the entire US population (including households and group quarters). You can get only the most general population characteristics from the decennial Census. The Census aids in apportioning congressional districts and determining federal funding for local programs. The main purpose is a relatively simple count of population.
The American Community Survey (ACS) is conducted on a rolling basis. It has a lot more in common with survey estimates rather than a complete population count. Only some people are surveyed at any given time -- about 1% of the US is surveyed each year. There are 1-year and 5-year estimates available. They have strengths and weaknesses in regards to geography size, timelineness, statistical error, and ease of interpretation.
| Decennial Census | ACS 1-Year (incl. Supplemental Tables) | ACS 5-Year | |
| Data Collection | Point-in-time (April 1 of years ending in 0) | 12 month continuous survey | 60 month continuous survey |
| Example | 2010 Decennial Census | 2017 ACS 1-year estimate | 2013-2017 ACS 5-year estimate |
| Available Geographies | Census block and larger |
Geographies with populations >65,000 Note that Supplemental, "high-level" tables are available for populations with >20,000 people (e.g., City of Charlottesville) starting in 2014. |
Census tract and larger |
| Design | Counts every person | Smaller sample size | Larger sample size |
| Margin of Error | Smallest margin of error, i.e. very reliable estimates | Highest margin of error, i.e., least reliable estimates | Margin of error smaller than 1-year estimate, i.e., more reliable than 1-year estimates |
| Best used when | When you are only interested in general population characteristics (count; age, sex, race) | Currency is more important than precision; when you are working solely with large population geographies; when you need more-easily interpreted population data | Precision is more important than currency; When you are working with less populous geographies (small-sized geographies or rural areas) |
| Drawbacks | Only available every 10 years (data age quickly); only most general population characteristics | Not available for less-populated geographies; high margin of error | 5 year estimate is harder to interpret; margin of error can still be high for small geographies |
Which survey should you use? Ask yourself these questions:
- What population characteristics am I interested in?
- If you only want the most general population counts and characteristics (age, sex, race), your best bet is the Decennial Census (or official Population Estimates for county geographies and larger for intercensal years).
- If you want more descriptive population characteristics (e.g., education, income, nativity), you have to work with the ACS.
- What type of geography am I interested in?
- If you are interested in larger geographies (states, larger cities/MSAs, or populous counties), the ACS 1-year is a good choice.
- If you are interested in anything but the largest geographies, you will have to choose the ACS 5-year estimates. For example, if you want smaller geographies like census tracts or all counties in Virginia (some of which are rural and have smaller populations), you will have to work with the 5-year estimates.
Understanding and Using American Community Survey Data: What All Data Users Need to Know
This handbook from the Census is an excellent reference for understanding the ACS:
- Understanding and Using American Community Survey Data: What All Data Users Need to KnowThis handbook provides an overview of the ACS to help data users understand the basics of the survey, how the data can be used, how to judge the accuracy of ACS estimates, and how to access ACS data. It also includes some recent case studies that show how ACS data are being used to help address important policy and program issues. Links to additional ACS resources, including technical documentation for more advanced users, are included throughout the handbook.
Table of Contents:
1. Understanding the ACS: The Basics
2. Geographic Areas Covered in the ACS
3. Understanding and Using ACS Single-Year and Multiyear Estimates
4. Making Comparisons with ACS Data
5. Accessing ACS Data
6. Case Studies Using ACS Data
7. Understanding Error and Determining Statistical Significance
8. Calculating Measures of Error for Derived Estimates
9. Differences Between the ACS and the Decennial Census
10. Using Dollar-Denominated Data
11. Measures of Nonsampling Error
12. Glossary