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Explainer: How the Census Gathers Data on a Dizzying Array of Facts, Figures

The real work is done by the American Community Survey, which builds up a database of everything from homes without kitchens to income of occupants

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The U.S. Census Bureau is the premier source of statistics about American life, ranging from basic information like the number of people living in a given geographic area to nuanced data such as how many housing units do not have full kitchen facilities.

This information now comes almost exclusively from the American Community Survey, which collects and produces information on demographic, social, economic, and housing characteristics about the nation's population every year.

The ACS doesn’t just collect information. It is also used to distribute some $400 billion in federal aid to states and communities each year for infrastructure and services.

History of the ACS: The U.S. Constitution requires the federal government to conduct a population count every 10 years to ensure that membership in the U.S. House of Representatives is properly proportioned among the states and among areas within the states and that the makeup of the Electoral College best reflects that of the country. Every decade beginning in 1790, such a count has been done. Since 1902, this has been the job of the U.S. Census Bureau.

The bureau's primary task used to be collecting data every 10 years. Every household was asked to fill out a form to capture basic information about the number of people in it. A certain proportion were given the so-called long form that asked for much more detailed information; the bureau used that to calculate estimates of more specific topics. After the 2000 Census, the bureau decided to change the format to collect that more detailed information each year through what it called the American Community Survey.

What it measures: The ACS provides data on dozens of topics. For people, it gathers information on age, ancestry, citizenship, disability, education, fertility, public assistance, grandparents as caregivers, health insurance, Hispanic origin, income, occupation, unemployment, language, marital status, migration, military service, race, and sex. For housing, among the topics it measures are bedrooms, computer use, farms, heating fuel, value of units, kitchens and plumbing, rent and mortgage, monthly costs, telephones, vehicles, and age of structure.

How the ACS is conducted: Every year, the U.S. Census Bureau contacts over 3.5 million households across the country to participate in the ACS. That's essentially 1 of every 38 households. People receive a survey they can fill out on paper and mail back, online or by phone. The questionnaires are sent out monthly, so the survey provides a snapshot of people and households over the course of a year, rather than on a given day -- when the decennial census counts people. The Census Bureau selects a random sample of addresses to be included. No address should be selected more than once every 5 years. The bureau mails questionnaires to approximately 295,000 addresses a month across the United States, which has more than 180 million addresses. Participation in the ACS, as in the decennial census, is required by law.

How it is used; Data collected by the survey is used for informational purposes, planning, and aid distribution.

The Brookings Institute reported that about $3 of every $10 in federal assistance is given using data from the ACS. Most of that is money for highway infrastructure and for Medicaid and other assistance to low-income households.

Federal agencies also use the information to assess programs. For instance, the federal labor and justice departments and the U.S. Equal Employment Opportunity Commission use ACS estimates to enforce employment antidiscrimination laws. The U.S. Department of Veterans Affairs uses the data to evaluate the need for healthcare, education, and employment programs for those who have served in the military. Federal education officials rely on ACS estimates to develop adult education and literacy programs.

Additionally, state and local agencies use the data for planning purposes -- the need for new roads or schools, or for emergency disaster-assistance preparation. Businesses use the data as part of their market-research efforts and to help determine where to locate new facilities. Researchers and journalists use the data to better understand and report on demographic, economic and housing trends.

How accurate is it: Using statistical models, census officials select addresses to represent other households in the surrounding community. The sample is designed to ensure good geographic coverage to produce an accurate picture of the community’s people and housing by surveying a representative sample of the population. Data is reported with sampling errors so users can judge how suitable it is for a particular purpose.

Depending on the size of the geographic area, data may only be reported as an average over a period of years. Single-year estimates are available for states and counties, but not for municipalities with a population of fewer than 65,000 because of accuracy concerns. For these smaller areas, only five-year estimates of population and housing characteristics are available.

Where is it: All ACS data is available on the Census Bureau's website through its American FactFinder tool.

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