Artikel Jurnal
Horne, M.R.
Granada, 1981
Buku Teks SO
editor, Roger L. Brockenbrough, Frederick S. Maerritt
McGraw-Hill, 1994
Buku Referensi
edited by Roger L. Brockenbrough, Frederick S. Merrit
McGraw-Hill, 1999
Buku Referensi
Lothers, John E.
Prentice-Hall, 1965
Buku Teks SO