1. Gibson, E. J., Osser, H., Schiff, W., & Smith, J. An analysis of critical features of letters, tested by a confusion matrix. In: A basic research program on reading. Cooperative Research Project No. 639. U.S. Office of Education, 1963.
2. Miller, L. K. Visual and auditory similarity ratings for capital letters. Unpublished manuscript. University of Illinois, no date.