The U.S. Census Bureau aims to count each person once—and only once—in the decennial census. It does that by determining how many people live at a every residential address.

But how does it count people with more than one address? Or military personnel deployed overseas? Or people incarcerated in correctional facilities? Or those who live in different places at different times of the year or who don’t have an address at all?

The answer is… it depends! And the answer matters because where people are counted can have far-reaching implications, from political representation to local infrastructure and services.

The Census Act of 1790 established—and every U.S. census has been based on—the concept of “usual residence,” which is the place where a person lives and sleeps most of the time. A usual residence is not always the same as a legal residence, voting residence, or even the location where a person prefers to be counted. Even so, identifying someone’s usual residence is pretty straightforward for most people. But there are special cases.

Census Residence Rules Address a Variety of Living Situations

To handle special cases, the Census Bureau has established residence rules to ensure that each person appears in the decennial count only once. For example:

College students living in student housing are counted as being residents there rather than at their parents’ or guardians’ home.

Hospital patients, including newborn babies, are counted at the residence where they’ll live and sleep most of the time once they’ve been discharged.

People who are living at a shelter, such as domestic violence and homeless shelters, are counted at the shelter.

People in temporary group living quarters established for victims of natural disasters are counted at the residence where they live and sleep most of the time. If they don’t have a usual home elsewhere, they’re counted at the facility.

U.S. military personnel assigned to U.S. military ships and vessels with a U.S. homeport on Census Day are counted at the onshore residence where they live and sleep most of the time. If they have no onshore residence, they’re counted at their vessel’s homeport.

People with seasonal homes or who have transitory living arrangements (such as RV parks and carnivals) are counted at the residence where they live and sleep most of the time. If they have no usual residence, they’re counted at their location on Census Day.

Foreign citizens are considered to be living in the United States if, at the time of the census, they’re living and sleeping most of the time at a U.S. residence. The foreign resident population includes legal permanent residents, foreign students in the United States on student visas, foreign diplomats and embassy staff, and other foreign citizens who reside in the United States on Census Day. However, citizens of foreign countries visiting the United States on a vacation or for business trip are not counted in the census.

The full list of residence rules for the 2020 Census published in February 2018.

New Rule for Deployed Military Personnel

Many of the 2020 residence rules remain unchanged from the 2010 Census count, but some changes in 2020 will affect where people are counted, which in turn could affect reapportionment—the process of allocating seats in the U.S. House of Representatives based on each state’s share of the U.S. population.

A key change to residence rules will affect where tens of thousands of overseas-deployed military personnel are counted. In a break from prior practice, the 2020 Census will count deployed personnel at their usual residence rather than at their “home of record” (the address the servicemember provided when they entered military service).

Consider the example of a sailor who enlisted in the Navy in Des Moines, Iowa, in 2011 and who is stationed in San Diego, California, in 2020. The sailor has leased an apartment in San Diego but is deployed to the Persian Gulf on Census Day. Under prior rules, the sailor would have been counted overseas and listed (only) in the congressional reapportionment count for Iowa. In 2020, based on the revised rules, the sailor will be counted in San Diego—for purposes of reapportionment and in all other data, including redistricting files and other local tabulations. This shift in residence rules is intended to better reflect the usual residence of deployed personnel and implications for local infrastructure and services, like hospitals and schools, associated with that population.

Deployments from a given community can range from a handful of personnel to several thousand, and deployments can be unpredictable, so the effect of this rule change on population counts will be unknown until after the 2020 Census is complete.

The change to recording deployed personnel at their usual residence rather than home of record will likely result in substantial population increases in communities near major military installations such as Fayetteville, N.C.; Jacksonville, N.C.; Killeen, Texas; Nashville, Ky.; Norfolk, Va.; San Diego, Calif.; and Tacoma, Wash.

Rule for Personnel Stationed Overseas Remains Unchanged

Servicemembers and federal civilian employees and family members who are deployed or stationed overseas for extended periods of time will continue to be counted as residing in their home state for apportionment purposes only for. the 2020 Census.

Residence Rule for People in Correctional Facilities Raises Concerns

People in correctional facilities have been, and will continue to be, counted in their location (jail or prison) on Census Day. But this rule has created controversy.

During the public comment period for the proposed residence rules for 2020, a substantial number of people (nearly 78,000) protested the rule for correctional populations, urging that prisoners be counted at their home or pre-incarceration address because of the temporary nature of most incarcerations. In 2017, about half (48 percent) of the 2.2 million people living in adult correctional facilities lived at a different residence the year before, and about 30 percent (more than 650,000 people) were incarcerated in a county different from their residence the prior year. Critics argue that counting the correctional population at the facility rather than at a pre-incarceration address inflates the electoral weight of voters in the districts with correctional facilities while diminishing the weight in other areas.

The Census Bureau notes that the existing rule aligns with the concept of usual residence. However, in acknowledgment of concerns about the correctional residence rule, the Census Bureau plans to provide additional population detail, including population counts for correctional facilities, when it publishes redistricting data. While the residence rule has not changed, data will be available for use in redistricting efforts.