Knapp, Rebecca Grant
John Wiley & Sons, 1978
Buku Teks
edited by Michael S. Lewis-Beck
Sage, 1993
Buku Referensi
Michael S. Lewis-Beck, editor
Sage Publications, 1993
Buku Referensi
Leabo, Dick A.
Richard D. Irwin, 1968
Buku Teks SO
Squire, Jessie E.
Mosby, 1973
Buku Teks