edited by Gale A. Mattox and John H. Vaughan
Westview Press, 1989
Buku Teks SO
Risjord, Norman K.
Prentice-Hall, 1985
Buku Teks SO
Stuart, Graham Henry, 1887-
Appleton-Century-Crofts, 1955
Buku Teks SO
Board of Trusless, 1960
Buku Referensi
Washington U.S. Department of Health, Education, and Welfare 1955
Buku Teks