"Population-Oklahoma"(PDF). U.S. Census 1910. U.S. Census Bureau. Archived(PDF) from the original on February 24, 2019. Retrieved November 22, 2013.
"Population-Oklahoma"(PDF). 15th Census of the United States. U.S. Census Bureau. Archived(PDF) from the original on June 9, 2011. Retrieved November 27, 2013.
"Number of Inhabitants: Oklahoma"(PDF). 18th Census of the United States. U.S. Census Bureau. Archived(PDF) from the original on September 12, 2015. Retrieved November 22, 2013.
Savage, Cynthia (2009). "Duncan". Encyclopedia of Oklahoma History and Culture (online ed.). Oklahoma Historical Society. Archived from the original on April 2, 2015. Retrieved March 30, 2015.
"Find a County". National Association of Counties. Archived from the original on July 12, 2012. Retrieved 2011-06-07.
Savage, Cynthia (2009). "Duncan". Encyclopedia of Oklahoma History and Culture (online ed.). Oklahoma Historical Society. Archived from the original on April 2, 2015. Retrieved March 30, 2015.
"Population-Oklahoma"(PDF). U.S. Census 1910. U.S. Census Bureau. Archived(PDF) from the original on February 24, 2019. Retrieved November 22, 2013.
"Population-Oklahoma"(PDF). 15th Census of the United States. U.S. Census Bureau. Archived(PDF) from the original on June 9, 2011. Retrieved November 27, 2013.
"Number of Inhabitants: Oklahoma"(PDF). 18th Census of the United States. U.S. Census Bureau. Archived(PDF) from the original on September 12, 2015. Retrieved November 22, 2013.