The 2010 United States census was the 23rd United States census. National Census Day, the reference day used for the census, was April 1, 2010. The census was taken via mail-in citizen self-reporting, with enumerators serving to spot-check randomly selected neighborhoods and communities. As part of a drive to increase the count's accuracy, 635,000 temporary enumerators were hired. The population of the United States was counted as 308,745,538, a 9.7% increase from the 2000 United States census. This was the first census in which all states recorded a population of over 500,000 people as well as the first in which all 100 largest cities recorded populations of over 200,000.