London : William Clowes and Sons Ltd
Majalah, Jurnal, Buletin
Smith, L. Glenn
Allyn and Bacon, 1973
Buku Teks
edited by Louis G. Locke, William M. Gibson and George W. Arms
Holt, Rinehart and Winston, 1967
Buku Teks SO
Chruden, Herbert J.
South-Western, 1984
Buku Referensi
edited by Keith Davis and William G. Scott
McGraw-Hill, 1959
Buku Teks