National Geographic Book Service.
National Geographic Society, 1987
Buku Referensi
Kolevzon, Edward R., author
Allyn and Bacon, 1959
Buku Teks
American Society for Training and Development Research
American Society for Training & Development, 2012
eBooks
Quesenbery, Whitney, author
Elsevier, 2012
Buku Teks
Eagleton, Clyde
New York University press, 1945
Buku Teks