United States Census Bureau
Legal mandate
Article One of the United States Constitution (section II) directs the population be enumerated at least once every ten years and the resulting counts used to set the number of members from each state in the House of Representatives and, by extension, in the Electoral College. The Census Bureau now conducts a full population count every 10 years in years ending with a zero and uses the term "decennial" to describe the operation. Between censuses, the Census Bureau makes population estimates and projections.
United States Census Bureau-4