Cowles, 1996
Buku Referensi
Townsend, A. L.
IKIP Semarang Press , 1986
Buku Teks SO
Woodson, R. Dodge
McGraw-Hill, 1994
Buku Referensi
Church, James C.
McGraw-Hill, 1979
Buku Referensi
Spain, Bryan
CRC Press, Taylor & Francis Group, 2009
Buku Teks SO