The Commerce Department’s decision to restore a citizenship question to the census beginning in 2020 is prompting concerns about curtailing participation and possibly undercounting people living in the United States.

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WASHINGTON — The United States census is so much more than just a head count. It is a snapshot of America that determines how congressional seats are apportioned, how state and federal dollars are distributed, where businesses choose to ship products and where they build stores. To do that properly, the count needs to be accurate.

The Commerce Department’s decision to restore a citizenship question to the census beginning in 2020 is prompting concerns about curtailing participation and possibly undercounting people living in the United States, particularly immigrants and minority groups who are expressing discomfort with answering questions from census workers.

Here are several of the commercial, political and research efforts that depend on accurate census data:

Divvying up seats in Congress, legislatures and more

The Constitution requires the government to enumerate the number of people living in the United States every 10 years, and to use that data to apportion the seats in Congress among the states. The calculation is based on total resident population — which means citizens and noncitizens alike — and it generally shifts power between the states once a decade, in line with population and migration trends.

States including Texas, Florida, Colorado and Oregon are projected to gain seats after the 2020 numbers are in. Illinois, Ohio, New York and West Virginia are among the states expected to lose seats. An undercount could shift those projections.

Lawmakers also use census data to draw congressional district boundaries within states, an often-controversial process that can help decide partisan control of the House. Census data also underpin state legislative districts and local boundaries like City Councils and school boards.

Handing out federal and state dollars

The federal government bases a large amount of its spending decisions on census data. Researchers concluded last year that in fiscal 2015, 132 government programs used information from the census to determine how to allocate more than $675 billion, much of it to programs that serve lower-income families, including Head Start, Medicare, the Supplemental Nutrition Assistance Program, Pell Grants for college and reduced-price school-lunch programs. Highway spending is also apportioned according to census data.

Influencing business decisions

To sell products and services, companies large and small need good information on the location of potential customers and how much money they might have to spend. The census provides the highest-quality and most consistent information on such items, and businesses have come to depend on it to make critical choices.

Census data help companies decide where to locate distribution centers to best serve their customers, where to expand or locate stores and where they have the best chance of seeing a high return on investment.

Planning for health and wellness

Scientists use census data to understand the distribution of diseases and health concerns such as cancer and obesity across the U.S. population, including drilling down to race and ethnicity to identify health patterns across demographics.

Public-health officials then use the data to target their interventions in at-risk communities. Inaccurate census data could lead public-health officials to invest in solving a problem that does not exist — or worse, to overlook one that does.

Gaming out Social Security

An undercount in the census could also impact forecasts about Social Security payouts, which are already increasing as a share of the federal government’s revenue.

When Congress plans for the costs of the country’s Social Security needs, lawmakers rely upon demographic projection about the population’s future: the number of children expected to be born, the number of people expected to die, and the number of people expected to immigrate.

If baseline data regarding the current population are inaccurate, future projections could be skewed, causing financial challenges down the line.