Zipf, George Kingsley, author
Hafner, 1965
Buku Teks
Hawley, Amos H., author
The Ronald Press, 1950
Buku Teks
Getis, Arthur, author
Macmillan, 1985
Buku Teks
The Association for Supervision and Curriculum Development, 1978
Buku Teks
Watson, Richard A., author
Harcourt, Brace and World, 1969
Buku Teks