Artikel Jurnal
editor by Celeste West; Elizabeth Katz
Booklegger Press , 1972
Buku Teks
Wilson, George P., author
F.S. Crofts, 1947
Buku Teks
Reader's Digest Service, 1987
Buku Teks
Estep, Tora, author
American Society for Training & Development, 2006
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