References: Lutz, M. C., & Chapanis, A. (1955). Expected locations of digits and letters on ten-button keysets. Journal of Applied Psychology, 39, 314-317. Deininger, R.L. (1960). Human factors engineering studies of the design and use of pushbutton telephone sets. Bell System Technical Journal, 39. 995–1012.

Written by Juanita N. Baker, Ph.D.