A decennial census for the purpose of congressional reapportionment is mandated by the U.S. Constitution. The first census was held in 1790 and counted only heads of household. Over time, the census has expanded to count every person in the U.S., including information on their age, race, ethnicity and more. While there may be portions of the population that go uncounted, it is the closest we have to complete demographic and economic data on the U.S. population.
Questions change from census to census (sometimes dramatically), which means that the statistics available change from decade to decade. The easiest way to find out what information is available for a specific census is to look at the census questionnaires. If the question wasn’t asked, the information isn’t available. It’s that simple. For copies of the original census questionnaires since 1790, see the Census Bureau publication Measuring America: The Decennial Censuses from 1790 to 2000.
For most of the 20th century, the decennial census included a “short form” with questions answered by every household in the country as well as a “long form” answered by about 1 in 6 households. Questions on the short form (age, race, etc.) are the basis for the census 100% data (available in Summary file 1 & 2). Questions on the long form (education, income, etc.) are the basis for the census sample data (available in summary file 3 & 4). The 2010 census did not include a long form; instead, sample data previously collected on the long form is now collected regularly through the American Community Survey (ACS).
The three resources below are the most comprehensive collections of decennial census/ACS statistics:
This guide is adapted from UC San Diego's Census Research Guide by Kelly L. Smith.