Anderson, Kristin, author
American Management Association, 1992
eBooks
Frey, James H., author
Sage Publications, 1983
Buku Referensi
Toepoel, Vera, author
SaGE Publications , 2016
Buku Teks
Marion Wesley Todd
Macmillan, 1934
Buku Teks
United Nations-ESCAP , 1985
Buku Teks