American Community Survey basics

What is ACS?

Every 10 years since 1790, Congress has authorized the government to conduct a national census of the
US population, as required by the US Constitution. In the twentieth century, the questions were divided between a short and long form. The short form was conducted on a 100% sample of the US population, while the long form was done on a 1-in-6 sample of the population and consisted of a much wider selection of questions.

After the 2000 Census, this long form became the ACS. Sent to approximately 3.5 million addresses per year, it is the largest household survey that the Census Bureau administers. Unlike census which is conducted once every ten years, ACS is conducted throughout the entire year, and while census is considered to be a snapshot of the population, ACS is much more like a video with its continually updated data.

How are ACS data collected?

The Census Bureau selects a random sample of addresses to be included in the ACS, and mails questionnaires to approximately 295,000 addresses a month across the United States. ACS forms are not mailed to specific people, but rather to specific addresses. The sample is designed to ensure good geographic coverage and does not target individuals. The survey takes place all year and interviews, where necessary, are conducted by permanent staff – not the temporary workers who interview for decennial census.

ACS comes in two releases: 1-year and 5-year estimates. 3-year estimates have been discontinued in 2013. The 1-year file is released for areas of at least 65,000 in population, the 3-year file was released for areas of at least 20,000, and the 5-year file for all of the areas for which the long form was released (block-groups, tracts and higher). 

What data can you expect to find in ACS?

ACS covers a lot wider selection of topics than the decennial census. You can find demographic information, such as age, sex, Hispanic origin, race, and relationship to householder; social characteristics, such as marital status, fertility, ancestry, place of birth, citizenship, year of entry, educational attainment, veteran status, and disability; economic characteristics, such as income, food stamps benefit, labor force status, place of work, journey to work, vehicles available, industry, occupation, and class of worker; housing data, such as year structure built, units in structure, rooms, bedrooms, house heating fuel, telephone service available, and farm residence; financial characteristics, such as tenure, housing value, rent, and selected monthly owner costs.

What is the data used for?

The ACS is taken every year, to provide more consistent ongoing information and to allow the census to focus on counting the population. These data are used by many public-sector, private-sector, and not-for-profit stakeholders to allocate funding, track shifting demographics, plan for emergencies, and learn about local communities. Businesses use ACS estimates to inform important strategic decision-making. For example, someone scouting a new location for an assisted-living center might look for an area with a large population of seniors and a large proportion of people employed in nursing occupations. If a person wants to see how they compare with their neighbors or find a new place to live, they can look to the ACS to provide a wealth of information. The ACS provides useful statistics about the median income of an area, the median age of the residents, the median house value, and monthly household expenses.

There are certain drawbacks that come with the data — for instance, the five-year file includes data from both before and after the financial crisis of 2007-08. As was the case with the long form, the community survey is a sample and so subject to sampling error. The confidence interval is often expressed as percent or number plus or minus another number that defines the interval. On the other hand, the benefit is that even for small areas data never are more than a few years out of date. In sum, despite some differences between the American Community Survey and the old census long form data, the survey is in most ways superior. Not only is the data released more often and in a more timely fashion, but the numbers may be more accurate since the survey is conducted by a permanent interview staff. 

When to use 1-year, 3-year, or 5-year estimates

The ACS provides many advantages over the information collected in the past through the decennial census long-form samples. The main benefits of the ACS are timeliness and access to annual data. The ACS will deliver useful, relevant data, similar to data from previous census long forms, but updated every year instead of every 10 years. However, the main challenge for ACS data users is understanding and using multiyear estimates.

1-year estimates

Best used when you're looking for currency and not precision, want to analyze smaller population and smaller geography.

3-year estimates

Best used when looking for more precise data, analyzing smaller populations and smaller geographies.

5-year estimates

Best used when looking for precision and not currency, analyzing very small populations, tracts and other smaller geographies.