Herrmann, Paul
Harper & Brothers , 1956
Buku Teks
Spence, Catherine Helen
Hale & Iremonger, 1988
Buku Teks
Spence, Jonathan D.
Norton Company, , 1990
Buku Teks
Spence, Catherine Helen
Penguin Books, 1984
Buku Teks
Cohen, William A., 1937- , author
American Management Association, 2008
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