The U.S. Constitution requires the census to be taken because the number of representatives each state has in the House of Representatives is based on the state's population. The census is taken every 10 years (also specified by the Constitution). While very little information is really necessary to meet the constitutional mandate (basically just population counts), the census has always gathered additional information useful for government policy decisions, planning, and other similar needs. This extra information is very valuable for genealogy and historical research, as well as many other types of research.
The census is a huge undertaking because every person is supposed to be counted in a relatively short period of time. For past censuses, census takers (also called "enumerators") were temporarily hired all over the country to visit each home, ask the people there for the required information, and record the information by hand. Each enumerator was assigned a geographic area to cover, sized appropriately so that they could cover the whole area during the census period.
The most recent census (2010) relied primarily on mail-in forms, plus about 635,000 enumerators to spot-check specific areas. The next census (2020) is expected to make heavy use of electronic forms submitted over the internet.
Once census information is collected, the Census Bureau organizes the forms, tallies numbers, and calculates various statistics. Tabulating statistics by hand was a huge task that required a large staff and could take several years. Tabulating machines were developed for use with the 1890 census. Better machines and eventually modern computers were used for later censuses.
Aggregate statistical information is made available by the Census Bureau as it becomes available, but the details about individuals are kept private for 72 years, after which the census is made public by the National Archives and Records Administration (NARA). The 1940 census was made public in 2012, and the 1950 census will become public in 2022.
Original census forms no longer exist for any of the censuses that have been made public. After being microfilmed for archival, the original forms were destroyed. This is unfortunate because a significant number of archival microfilm images are of poor quality (incorrect exposure, poor contrast, out of focus or otherwise blurred, etc.)
The accuracy of the census is limited by the process itself, by the individuals providing information in behalf of their household, and by the enumerators recording the information. Since it takes several days or weeks to enumerate a census, people who traveled during a census period may have been missed altogether or even counted in more than one place. The person reporting information may have remembered incorrectly, guessed, or even intentionally lied about themselves or others who lived there. The enumerator may have heard wrong, accidentally wrote the wrong thing, or made incorrect assumptions about spelling or other details. If nobody was home, the enumerator was allowed to collect the information from someone living nearby, who might be less accurate. Enumerators usually used draft census pages to collect the information at the homes, then copied the information onto new pages for official submission, which sometimes introduced copy-errors.
In cases where nobody could be found at home, the enumerator might not have found anyone else who could answer for them. Or homes that were off the beaten path or unknown to the enumerator may have been accidentally skipped.
Occasionally, a page, a few pages, or even entire towns, counties, and states of a census have been lost. Most dramatically, the entire 1890 census was destroyed. The census volumes were heavily damaged during a fire at the Commerce Department Building (where they were stored) in 1921. The census records were later marked for disposal and then destroyed during the 1930s despite objection from many historians and large genealogical organizations. A few surviving fragments of the census were later found, but they total only about 6,160 names.
The questions asked on each census vary. The first census (in 1790) only recorded the names of the heads of families and how many family members were in each of a few specific categories (free white males 16 and older, free white males under 16, free white females, all other free persons, and slaves). Additional categories, then details of other types, were added over the next few censuses.
The 1850 census was the first to collect names for all individuals instead of just the heads of families, and also used a pre-printed census form. For each person, their name, age, sex, color, profession (males only), value of real estate owned, place of birth, whether married or attended school within the year, whether illiterate, and whether "deaf and dumb, blind, insane, idiotic, pauper, or convict" were recorded.
The census form continued to evolve over time. By 1930, it also asked about the birthplace of each person's parents, native language, citizenship status, veteran status (and which war), street name and house number, and even whether the family had a radio.
Some censuses asked everyone a set of general questions, plus additional questions from a smaller subset of the population. Recent censuses, for example, have used both a "short form" with fewer questions, and a "long form" (given to a sample of the population) to gather additional details for certain statistics useful to the government.
We have focused on the population information collected in the U.S. Census. The government has also collected other types of census information over the years. In some censuses there are census schedules (forms) other than population schedules. And some censuses apart from the 10-year census have been taken to collect various types of data. For example, more in-depth agricultural information, details about veterans and pensioners, Native Americans, unemployment data, and various other types of information has been collected at different times. In addition, many states have taken their own censuses.