1. Anderson, Vernon E., and Gruhn, William T.Principles and Practices of Secondary Education(2nd ed.). New York: The Ronald Press Company, 1962, pp. 87, 91.
2. Bent, Rudyard K., and Kronenberg, Henry H.Principles of Secondary Education(4th ed.). New York: McGraw-Hill Book Company, Inc., 1961, pp. 215–219.
3. Cassel, Russell N., and Coleman, Jack C. “Are the Causes of Dropouts Excuses?”The Bulletin of the National Association of Secondary-School Principals, 46: 63, 64 (November, 1962).
4. Cronbach, Lee J.Educational Psychology(2nd ed.). New York: Harcourt, Brace and World, Inc., 1963, pp. 220–21.
5. Crow, Lester D., and Crow, Alice.Educational Psychology(New Revised Edition). New York: American Book Co., 1963, p.195.