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American Community Survey - Frequently Asked Questions

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What is the American Community Survey?

The American Community Survey (ACS) is an ongoing survey that provides data every year, giving communities the current information they need to plan investments and services. Information from the survey generates data that help determine how more than $400 billion in federal and state funds are distributed every year. To help communities, state governments and federal programs, the US Census Bureau collects data on:

What is the difference between the ACS and the Decennial Census?

The ACS is conducted every year to provide up-to-date information about the social and economic needs of your community. The ACS shows how people live- our education, housing, jobs and other characteristics. For example, results may be used to decide where new schools, hospitals and emergency services are needed.

The Decennial Census is conducted once every 10 years to provide an official count of the entire US population to Congress.

What is the difference between the 1, 3, and 5 year estimates, and which one should I use?

There are multiple differences between 1, 3 and 5 year estimates. The US Census Bureau provides in-depth guidance on the differences and on which estimates might be right for your purpose on their website.

What is a Margin of Error?

All data that are based on samples, such as the ACS, include a range of uncertainty. The Margin of Error (MOE) refers to sampling errors which occure when data are based on a sample of a population rather than the full population. MOE measures the precision of an estimate at a given level of confidence- 90% for the ACS. In the simplest of terms, this means that the Census Bureau is 90% certain that the difference between the population value and the sample estimate is less than or equal to the MOE.

Sampling error can be used to assess the statistical reliability of survey data. For any given area, the larger the sample and the more months included in the data, the greater the confidence in the estimate and the smaller the MOE.

Please visit the Understand Margins of Error page to view our in-depth MOE explanation with example.

Can I just ignore the MOE and present the data without it?

The State Demography Office strongly encourages data users to evaluate all data carefully in order to understand the true nature of the numbers. It is exciting for those using the data to have access to more up-to-date statistics, but this information must be used and interpreted with caution. Data users may be tempted to ignore or misuse the margins of error, however ignoring the MOE, especially when the data is being used in a decision-making process, can be extremely misleading.

What are all the data products available from the ACS?

Download a handy "cheat sheet" listing data products and the types of information available in each.

Can I compare ACS estimates from different years, or to Decennial Census Data?

1-year data: Generally, you can compare ACS 1-year estimates with Census 2000 and other ACS 1-year data. However, since ACS variables change over time, some areas and subjects must be compared with caution, or not compared at all.

3-year and 5-year data: Generally, you can compare ACS 3-year and 5-year estimates with Census 2000 data, however there are significant differences in the universe, question wording, residence rules, reference periods and the way in which the data are tabulated which can impact comparability.

General Guidance for comparing ACS multiyear estimates (from the US Census Bureau):

When comparing estimates for different areas, use the same period length for each estimate. This means you should not compare a 1-year estimate to a 3-year estimate. The Census Bureau discourages direct comparisons between estimates for overlapping periods. Instead, compare nonoverlapping estimates. This means we discourage you from comparing the 2006-2008 estimates to the 2007-2009 estimates. It is better for you to compare a 2005-2007 ACS estimate to a 2008-2010 ACS estimate. The strength of the ACS is in estimating characteristic distributions. We recommend users compare derived measures such as percent, means, medians and rates rather than estimates of population totals.

Additional guidance is available from the US Census Bureau.

Can I download a complete user's guide to the ACS?

The US Census Bureau provides multiple PDF Compass handbooks to help different user groups with specific how-to instructions and/or case studies. The Compass guides are available for many groups including general data users, PUMS data users, rural data users, state and local governments, and high school teachers, among others, and can be downloaded at the US Census Bureau website.