December 6, 2016
It’s already December, and that means it’s time for another data literacy post: this time, in honor of the upcoming American Community Survey (ACS) 2011-2015 5-Year Estimates release on Thursday, December 8th, we’ll be talking about what the ACS is, what topics it covers, how it can be used by members of the public, and what we use it for here at CCRPC.
What is the ACS?
The ACS is a data product from the U.S. Census Bureau. It is a series of estimates that cover a comprehensive range of topics and geographic areas, from the general to the highly specific. ACS data is available for free in detailed tables, area summary reports, and maps.
What data topics are included in the ACS?
The ACS covers a wide range of topics. Two of the major categories are demographics (e.g., age, sex, race, ethnicity, etc.) and economics (e.g., income, poverty, employment). Beyond those areas, there is also data on education, transportation behaviors, native and foreign-born populations, household and family topics, veteran status, populations having a disability, housing topics, and more.
Who can access and use ACS data, and where can I find ACS data?
Everyone! Data from the ACS, like data from other Census products, is public and free to use. Obviously, participants’ full responses are not released, since those include personal information like names and addresses. But once estimates are compiled into tables and published, anyone can go to Census.gov and access the data, and anyone can use the data as a source for projects, papers, presentations, or just general curiosity.
The user interface that allows access to most Census data is American FactFinder: users define their geographies of interest, pick their years of interest, and then get down to searching for their topic. If you’re not sure how to get started, the Census Bureau offers a number of tutorials on how to use American FactFinder, covering tasks from the basic to the advanced.
It should go without saying that, if you’re going to use ACS or other Census data, you should make sure you cite it. There is a recommended citation format for tables from American FactFinder outlined in a U.S. Census FAQ (and also, handily, at the bottom of this page, where we footnote some income figures); that’s what we use here at CCRPC when we use Census data, and we strongly encourage other data users to take the time to cite it right.
How often is the ACS produced and released?
The ACS is conducted on a rolling basis. Unlike the Decennial Census, where every household in the nation is asked to respond, only some households receive materials to respond to the ACS each month. Data collected from these sample populations is then extrapolated into the estimates about the total population.
Both datasets are released every year. In general, the new 1-Year Estimates are released in September of every year, and the new 5-Year Estimates are released in December.
What’s the difference between 1-Year Estimates and 5-Year Estimates?
There are actually several. The primary difference is the timeframe that the data covers. The 1-Year Estimates are applicable to the year named in their title: for example, the 2015 1-Year Estimates that were released in September are applicable to 2015. The 5-Year Estimates are applicable to the five-year period named in their title: for example, the 2011-2015 5-Year Estimates that will be released on December 8th cover the entire five-year period. Keep in mind that figures from the 5-Year Estimates are not the same as the figures from single years within that five-year period, and cannot be presented as such. Put simply, the 5-Year Estimates are a five-year average, while the 1-Year Estimates cover their single title year.
To offer a concrete example, let’s say that you’re looking for data on income, and you specifically want to know what percentage of households in Champaign County made between $25,000 and $34,999 in 2014. In that case, you should use data from the 2014 1-Year Estimates, and the figure you would find is 10.3% (+/-1.6). If you use the 2010-2014 5-Year Estimates, you’ll find a figure of 10.5% (+/-0.7). The figure from the 5-Year Estimates is slightly different, and it cannot be used to accurately represent the percentage of households in that income bracket in 2014. Both of these things are because the 5-Year Estimates include not only data collected in 2014, but also data collected in 2013, 2012, 2011, and 2010.
Also for this reason, you shouldn’t compare data from 5-Year Estimates datasets with overlapping years: for example, when the 2011-2015 5-Year Estimates dataset is released next week, you can’t compare its data with data from the 2010-2014 5-Year Estimates, because both datasets include the data collected in 2011, 2012, 2013, and 2014. If you want to compare 5-Year Estimates datasets, it’s only really doable with datasets with non-overlapping periods (e.g., comparing the 2005-2009 5-Year Estimates and the 2010-2014 5-Year Estimates, or comparing the 2006-2010 5-Year Estimates and the 2011-2015 5-Year Estimates).
Another major difference between the 1- and 5-Year Estimates datasets is the geographies they’re available in. The 1-Year Estimates include data from geographies with populations of 65,000 or more, while the 5-Year Estimates include data from all geographies in the United States. The limited number of geographic areas means that, even if you’re looking for data from a single year, it may be necessary to use 5-Year Estimates because your geography has a population of below 65,000, and has no 1-Year Estimates available. In that case, it’s important to know how to work with the 5-Year Estimates, and how to interpret and talk about data from that resource.
One more thing to keep in mind is that the margins of error of the 5-Year Estimates datasets tend to be smaller than the margins of error of the 1-Year Estimates datasets, because the 5-Year Estimates include more responses. In general, the larger the input dataset, the smaller the margin of error. This may be a reason to opt for using a figure from the 5-Year Estimates even when data is available from your geography of interest in the 1-Year Estimates.
Finally, data from 1-Year Estimates can’t be compared to data from 5-Year Estimates, even if they’re the same figure drawn from the same table and covering the same geography – the difference in their respective time periods makes the datasets not comparable.
Why does ACS data include margins of error when Decennial Census data doesn’t?
As we noted above, and have mentioned in previous blog posts, ACS data are estimates, and estimates can always be off, either by a small amount or by a massive amount. The margins of error reflect that range of possibilities: that the estimate is X, but that the actual number could be as high as X+Y, or as low as X-Y. For more information on margins of error, check out our last data literacy post, which covers them in detail.
On the other hand, Decennial Census data are exact counts. Decennial Census surveys are now much shorter and have much less detail than the ACS, but they are point-in-time counts instead of estimates based on a population sample. Counts don’t have margins of error because they’re assumed to be absolutely correct (give or take some non-responding households) – they don’t need them.
How does CCRPC use ACS data?
The ACS is a great resource for public agencies as much as it is for individuals. Having the wide variety of topics available on a consistent basis is a big part of what allows us to develop and maintain this website, its indicators and analysis, and the regional dashboard. We use ACS data in our own reports, like the Long Range Transportation Plan. We also use it to create reports for other agencies and organizations, and to respond to data requests from private individuals. Not every request or report uses ACS or Census data, but many of them do.
ACS data is necessary for our work beyond any specific plans, reports, and data requests. CCRPC is one of many public, service-providing agencies in Champaign County and around the country; for organizations like ours, knowing about the population we serve and represent, both in general and in detail, is fundamental to our operations. The ACS may not be a perfect resource, and may not be suited to every data need or project, but in many cases, it is the best and only data that is reliable, released on a regular schedule, and free to use. It has enormous value, to CCRPC and as a public resource.
For even more information on the ACS, check out the ACS homepage on the Census Bureau website. In closing, we’d just like to say that we love our Census data, and that the ACS is a great resource for a wide range of users and topics. So check it out! You’ll probably find something interesting, too.
 U.S. Census Bureau; American Community Survey, 2014 American Community Survey 1-Year Estimates, Table S1901; generated by CCRPC staff; using American FactFinder; (28 November 2016).
 U.S. Census Bureau; American Community Survey, 2010-2014 American Community Survey 5-Year Estimates, Table S1901; generated by CCRPC staff; using American FactFinder; (28 November 2016).