Chatburn, Robert L., author
Jones & Bartlett Learning, 2011
Buku Teks
Abbas Tashakkori, Charles Teddlie
SAGE Publications, 2010
Buku Teks SO
Harold A. Rothbart, editor in chief
McGraw-Hill, 1985
Buku Referensi
Lingaiah, K., author
McGraw-Hill, 1994
Buku Referensi
edited by Richard N. White, Charles G. Salmon
John Wiley & Sons, 1987
Buku Referensi