Conway, Mal
American Society for Training & Development Press, 2003
eBooks
Reeves, Tom Kynaston
McGraw-Hill, 1981
Buku Teks
edited by Sabine Hader, Michael Hader and Mike Kuhne
Springer, 2012
eBooks
Frey, James H.
Sage Publications, 1983
Buku Referensi
Foddy, William
Cambridge University Press, 1993
Buku Teks SO