Visits to the National Museum of Natural History in the U.S. 2001-2019

Because the Smithsonian museums are free, they cannot base our visit tallies on a number of tickets sold. Smithsonian security officers use hand clickers to count everyone entering the museums (in some buildings they count people leaving) through public entrances during the hours the museums are open. The counts sometimes include staff, as well as visitors who leave the museum and return. A person visiting three Smithsonian museums on any given day will be counted three times.
The source does not provide a date of publication. The date given is the date accessed.