University of Illinois, 1960
Buku Teks
Ely, Richard T., author
University of Wisconsin Press, 1964
Buku Teks
Ely, Richard T., author
Macmillan, 1924
Buku Teks
edited by W.L. Gibson, Jr, R.J. Hildreth, Gene Wunderlich
University of Nebraska Press, 1966
Buku Teks
Ratcliff, Richard U., author
McGraw-Hill, 1949
Buku Teks