1. J.H. Menning and C.W. Wilkinson, Communicating Through Letters and Reports, 4th ed. Homewood, Illinois, Richard D. Irwin , 1967, p. 3.
2. Wilbur Schramm, The Process and Effect of Mass Communication, Urbana, University of Illinois Press, 1965, p. 12.
3. F.L. Whitney, The Elements of Research, 3rd ed. New York, Prentice-Hall, 1950, p. 19.
4. M.R. Cohen, Reason and Nature: An Essay on the Meaning of Scientific Method, New York, Free Press, 1964, p. 83.