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USP 185A - Finding Census Data: Home

Decennial Census and American Community Survey: Overview

Held every 10 years since 1790, the Census is the most comprehensive demographic survey in the United States. Although the questions asked change from year to year, through 2000 there were always variables related to age, race and ethnicity, languages spoken, education, ancestry, income, home ownership versus renting, and more. In theory, the Census counts every person living in the United States. Although in reality there may be portions of the population that go uncounted, it is still the closest we have to complete demographic and economic data on the U.S. population.

As of 2010, there is no longer a decennial Census long form. A decennial short form, containing only questions such as age, race and ethnicity, sent to every household in the country. A longer questionnaire called the American Community Survey (ACS), released more frequently, covers questions previously appearing on the decennial long form.

Since 2005, the ACS - which includes approximately 50 questions - has been sent to 1 in 40 households in the U.S. Over a period of five years, this approximates the number of households that would have normally filled out a long form decennial Census. Starting in 2010, aggregates of five years of data will be released each year. This means that fresh demographic data about the country will be available every year instead of every 10 years. Although the ACS data is only sample data, it's a great improvement in the availability of recent demographic data.

An important point to keep in mind is that, due to differences between how data was gathered for the decennial census long form and for the ACS, you need to be very careful when making data comparisons between census years. A couple of good resources for getting a better understanding of this are available from the Oklahoma Department of Libraries and the Census Bureau.

Important Note: Tract-level data is not available for ACS 1-year or 3-year estimates.

What's in the Census?

Questions change from Census to Census, sometimes dramatically, which means that the statistics available change from decade to decade. For most of the twentieth century the Decennial Census has included a “short form” with questions answered by every household in the country, as well as a “long form” that is answered by about 1 in 6 households. Questions on the short form (age, race, etc.) are the basis for the Census 100% data (available in summary file 1 & 2). Questions on the long form (education, income, etc.) are the basis for the Census sample data (available in summary file 3 & 4).

The easiest way to find out what information is available for a specific Census is to look at the Census questionnaires. If the question wasn’t asked, the information isn’t available. It’s that simple. For copies of the original Census questionnaires since 1790, see the Census publication Measuring the Census: The Decennial Censuses from 1790 to 2000; the Census Bureau's Index of Questions site provides questions asked for decennial censuses from 1790-2010. The 2020 decennial questions are also available on the Census website. The Bureau also provides an archive of ACS questions asked.

There's also this very handy chart that shows which variables appear on the decennial Censuses from 1790-2000.

ACS estimates

Each year, the Census Bureau releases 1-, 3- and 5-year estimates based on information gathered in the ACS.

In deciding which estimate you want to use, you should consider the currency of data; the geographic size of your population; and the acceptable sample size/reliability of the data.  The Census Bureau chart below shows distinguishing features of the different estimates.

Tract-level data is not available for ACS 1-year or 3-year estimates.

Important note: if you wish to compare ACS estimates to earlier decennial Census data, please review these cautions first.


1-year estimates

3-year estimates

5-year estimates

12 months of collected data

36 months of collected data

60 months of collected data

Data for areas with populations of 65,000+

Data for areas with populations of 20,000+

Data for all areas

Smallest sample size

Larger sample size than 1-year

Largest sample size

Less reliable than 3-year or 5-year

More reliable than 1-year; less reliable than 5-year

Most reliable

Most current data

Less current than 1-year estimates; more current than 5-year

Least current

Best used when

Best used when

Best used when

Currency is more important than precision

Analyzing large populations

More precise than 1-year, more current than 5-year

Analyzing smaller populations

Examining smaller geographies because 1-year estimates are not available

Precision is more important than currency

Analyzing very small populations

Examining tracts and other smaller geographies because 1-year estimates are not available

Using Social Explorer

Generally speaking, I strongly encourage students to use Social Explorer instead of

Social Explorer is much easier to use than the Census Bureau's website and it includes historical census data - unlike You can use Social Explorer to run data reports on geographic areas as small as census blocks, and SE's mapping function can also help you identify census tracts back to 1950.

To access the database from off-campus, you will need to have your computer configured to use the UCSD VPN. If you need help with that, please refer to the remote access instructions. Note that you MUST select 2-Step Secured - Allthruucsd from the "Group" drop-down menu when authenticating.

To identify census tracts:

1. click “explore maps” in the left column

2. click "explore" button in the United States section

3. close out the "start tour" popup (or, feel free to take the tour!)

4. zoom into your geographic area. You can either use the search bar for this (e.g. North Park San Diego CA, then select appropriate zip code), or double click and reposition the map as you go. As you zoom in, you’ll eventually see details like street names and census tract numbers. Use Google maps or similar resource and the city's community profiles map as necessary to help identify neighborhood boundaries and streets. As you mouse over the Social Explorer map, you'll see census tract numbers appear. Make note of these for finding tract data.

5. to see a map for a different year (it defaults to the 2019 ACS 5 year), click the "Change Data" button on the top left. Select the desired year, then choose population > total population to create a new map. Repeat step 4 above.

To get data:

From the home page:

  • click "Tables" in the left column
  • select the decennial year or ACS year that you want to work with, then click "begin report"
  • use the drop-down menus to select your geography (e.g. nation, state, census tract, or place - use "place" for cities); use CTRL + click to select multiple geographies
  • after selecting your geography/geographies, click click "Add" and "Proceed to Tables" 
  • the available geographies will vary, depending on whether you're using decennial / ACS 5 year / ACS 3 year / ACS 1 year data

On the resulting page, you have a few options on how to proceed. The available tables will also vary, depending on whether you're using decennial / ACS 5 year / ACS 3 year / ACS 1 year data.


  • select the "premade reports" tab and then choose the "essentials" or "comprehensive" report option, then click add and show results


  • use the "list tables" tab to select individual tables of interest (note: there are different options depending on whether you use the "Social Explorer tables" or "Summary File 1" tables as your dataset. I generally recommend starting with the Summary File 1 tables.)
  • highlight the desired tables, then click add/show results


  • use the "search by keyword" to see available tables
  • enter your keyword (e.g. housing), then click the desired tables and add/show results