Cass, James, author
New York, Harper & Row Publishers
Buku Referensi
Frederick, Robert W.
USA Appleton 1947
Buku Teks
Berkeley University of California Press 1968
Buku Teks
Spanbauer, Stanley J.
ASQC Quality Press, 1993
Buku Teks
Mallinson, Vernon
London Heinemann 1960
Buku Teks