Census data can be gathered through direct enumeration or self-enumeration. A third method of counting a nation's population is to conduct a sample survey. Instead of enumerating every individual, a representative sample is queried. The higher the sampling rate, the more reliable the results will be. Sampling lowers the cost of the entire census and reduces the burden on individuals.

The United States Census has been conducted decennially since 1790, in years ending with zero. It is mandated by the Constitution, for the purpose of determining political representation in the House of Representatives. Congress is responsible for determining the method that will be used to conduct the Census. In 1954 all laws currently in force concerning the Census were consolidated into Title 13 United States Code where the powers and duties of the Census Bureau are named. Under this law, all U. S. residents are required to provide complete and truthful responses to Census inquiries, and the confidentiality of all responses is guaranteed.

The format and information available in the Census has varied over the decades. The first Census in 1790 asked only 6 questions and was used to gage the number of individuals available for military service in the new nation. The 1810 Census attempted to measure the output of the nation's manufacturing establishments and the 1820 Census was the first to inquire about occupational status of individuals. With each Census, the range of questions has increased. The 1840 Census added demographic questions including school attendance, literacy, occupation, and mental disability. The 1850 Census is considered to be the first to follow modern enumeration principles because it captured characteristics of individuals rather than entire families. Until 1870, results were tabulated with paper and pen on long tally sheets. The Census grew, and in 1890 a total of 13,000 possible questions could be asked by the Census takers. In the late 1800s, a rudimentary mechanical tallying machine was used and it was followed by the punched card system invented by Census employee Herman Hollerith.

The year 1940 marked the first use of sample surveys in the Census. In the 1960 Census mail-out questionnaires were used. Households were instructed to keep the completed forms until they were picked up by an enumerator. The 1970 Census carried self-enumeration further and 60% of all households were instructed to return their completed questionnaires by mail. In 1980, over 90% of the Census questionnaires were delivered by mail.

From a reasonably small number of questions, and the ability to cross-tabulate responses, many important public policy facts can be derived. The government relies on Census data for many other purposes besides Congressional reapportionment and redistricting. Billions of dollars of federal aid are distributed to states and local communities according to formulas based on decennial Census figures. Census schedules housed in the National Archives are copies of the actual forms filled out for every family or household. Those Census records more current than 72 years are completely confidential, but individuals can request official transcripts of their own Census responses.

The Census Bureau has used innovative technologies to manage information in the past. These include:

• 1890: Herman Hollereth uses punched cards to represent census data. This invention leads to his founding the company that will later become IBM;
• 1951: used the UNIVAC I computer to tabulate part of the 1950 census;
• 1953: developed a film optical sensing device for input to computers (FOSDIC), which was used perhaps beyond its useful life in processing 1960, 1970, 1980, and 1990 census questionnaires;
• 1980: developed computer-assisted interviewing software;
• 1986: created and distributed data files on CD-ROM;
• 1994: implemented the Census Bureau Website (http://www.census.gov)
• 1994: automated the Current Population Survey, using Computer-Assisted Personal Interviewing software to survey approximately 69,000 households monthly;
• 1996 initiated the American Community Survey Program;
• 1999: made available the initial implementation of the American FactFinder System, an Internet-based component of the Data Access and Dissemination System (DADS);
• 2000: conducted the Year 2000 Decennial Census including options to respond online.

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