This post explains the differences between the American Community Survey (ACS) and the Decennial Census and how those differences will impact your work.
Background: Long Form and Short Form
Remember back to the Census 2000 when some people were complaining about the Census form taking forever to fill out? Well, those people received the Census 2000 Long Form which was sent to about 18 million households and collected detailed demographic, economic and housing data. The rest of the US households received the simple Census 2000 Short Form, which was used to count the population and collect basic demographic data. So for the Census 2000, 2 forms were used: the Long Form and the Short Form.
Fast-forward to the Census 2010. The Census 2010 did not use the Long Form, only the Short Form. Enter the ACS. The American Community Survey (ACS) is an annual demographic survey of the United States. It provides the detailed demographic, economic and housing data that was once supplied by the Decennal Census Long Form.
|Count + Basic Data||Short Form||Census 2010|
|Detailed Data||Long Form||ACS 5 year estimates|
However, the ACS is only sent to 3 million households each year, a statistically small sample especially when compared to the Census 2000 Long Form which was sent to 18 million households in 1 year. So to produce the detailed demographic data, the ACS combines data from several years to produce multi-year estimates. For example, the ACS combines data collected from 2006 + 2007 + 2008 to get 3-year ACS estimates for geographies with at least 20,000 persons. ACS 5 year estimates, which will be out in December 2010, will provide data for geographies with fewer than 20,000 persons.
Similar Data; Different Methodologies & Format
The ACS and the Decennial Census provide the same types of data data–that is, demographic, economic and housing data for the US. However, the methodology of these 2 tools differs greatly.
|What is It?||Continuous demographic survey||Count of the population|
|Data||Average characteristics OVER TIME||Counts characteristics for a SPECIFIC DATE|
|How Often||Every 1 year (more timely)||Every 10 years (less timely)|
|Households||11 million households over 5 years||18 million households in 1 year|
|Sampling Error||Higher Error (1.75 percent higher than Census 2000*)||Lower Error|
|Accuracy||Less Accurate||More Accurate|
*[Revised: this information (and much of the blog post) is based on the Census Bureau's Brian McKenzie's awesome presentation here]
To sum up the table above, the ACS more timely but less accurate than the Decennial Census, because the data is collected over a series of years (instead of just 1 year), and fewer households are surveyed.
Because the ACS is less accurate, ACS data includes margins or error and is released at 90% confidence limits. Here’s an example of ACS data.
This means that we can say with 90% confidence that the number of Hispanic persons in Austin is between 258,268 and 265,076 (or 261,672 plus or minus 3,404). We can also say with 90% confidence that the percentage of Hispanic persons in Austin is between 34.6% and 35.4%.
How Does All of this Impact You?
- ACS demographic data is less reliable than Decennial Census data. It’s going to be more important than ever use ACS data as a starting point only and to support the data fieldwork, interviews, and data from other sources.
- Doing basic calculations with ACS data is going to require an elementary understanding of statistics. For example, calculating percentage change between Census 2000 data and ACS estimates will require basic statistics, because the numbers are not comparable (they are apples to oranges).
If you’re interested, let me know in the comments if a blog post on how to accurately calculate comparisons using Decennial Census data and ACS estimates would be helpful. Or you can check out the Census’ guidance on the topic. Any questions about the differences between ACS and Decennial Census data? Are there still points of confusion?
Right now, you can get ACS data for your projects in seconds with the Free Poverty Report beta. When the ACS 5 year estimates come out in December, this data will be part of the Cubit system.