A census was a statistical accounting of all the inhabitants of a region. They could be invaluable sources of information when performing genealogical research. (VOY: "11:59")
In 2269, James T. Kirk was taken aback by the apparent lack of life in the Malurian system, given that the last census had reported more than four billion inhabitants. (TNG: "The Changeling")
In 2367, Ardra provided the Ventaxians with guidelines for a full census of their population. (TNG: "Devil's Due")
While researching her ancestor Shannon O'Donnell, Kathryn Janeway preferred such "raw material" sources to various histories of 21st century Earth, as she found them to all be biased. (VOY: "11:59")