Population census

A population census is the organized enumeration or equivalent statistical accounting of the people in a specified territory at a specified reference time, including selected demographic characteristics. Universality and a common reference time distinguish the target of the count from the days on which information happens to be collected.^[ref]

For example, if interviews span two weeks, a person born after census night should not silently become part of the census-night population merely because the household is interviewed later. Rules about usual residence also prevent a student or traveler from being counted inconsistently across locations. The resulting counts supply denominators for rates and benchmarks for statistical sampling. Attempting complete coverage does not eliminate omission, duplication or classification errors; a census is a measurement procedure, not a guarantee of perfect counts.