Reed, William F. (September 5, 1994). "Early Master:Cal Hubbard". Sports Illustrated. Archived from the original on June 28, 2013. Retrieved April 7, 2013.
baseball-almanac.com
"Cal Hubbard obituary". The New York Times. October 18, 1977. Retrieved April 6, 2013 – via Baseball Almanac.
Reed, William F. (September 5, 1994). "Early Master:Cal Hubbard". Sports Illustrated. Archived from the original on June 28, 2013. Retrieved April 7, 2013.
"Robert Calvin Hubbard bio". The State Historical Society of Missouri via website. 2013. Archived from the original on November 9, 2012. Retrieved April 5, 2013.
"Robert Calvin Hubbard bio". The State Historical Society of Missouri via website. 2013. Archived from the original on November 9, 2012. Retrieved April 5, 2013.