The American Community Survey was created to gather detailed trend data for planning purposes on a more frequent basis than the decennial censuses. It is run every year and uses a quite different methodology from the decennial census long form. For this reason, while the subjects included in the ACS are very similar to the decennial long form subjects, data from the two surveys are often not directly comparable. Subjects are quite varied and include data on income, education (enrollment and attainment), employment and occupation, transportation to work, health insurance coverage, detailed housing characteristics like prevalence of complete kitchen and bathroom facilities and heating fuel, and more. Subjects are subject to revision (health insurance coverage was added to the 2009 questionnaire); check the technical documentation for specific subjects.
The ACS began full implementation in 2005 but group quarters were not included until 2006. The questionnaire is administered every month and the overall sample is accumulated over multiple years: geographies with more than 65,000 people are represented in the one-year (annual) data; geographies with between 20,000 and 65,000 people are represented in the three-year data; geographies with fewer than 20,000 people are represented in the five-year data. Data in the ACS, like the decennial long form, are available down to the Census block group level but estimates at this level may not be reliable given the smaller sample size of the ACS (12.5% before 2011; about 13.5% after 2011). Users should pay close attention to the Margins of Error published with the data—they can be quite large for small groups of population and small geographies.