1. Drucker Peter F., The Effective Executive (New York: Harper and Row, 1967), pp. 26–27.
2. Terry George R., Principles of Management, 4th éd. (Homewood, Ill.: Irwin, 1964), pp. 50–57.
3. Koontz Harold, O'Donnell Cyril, Principles of Management, 3rd ed. (New York: McGraw-Hill, 1964), pp. 27–28.
4. Fryer Douglas H., Henry Edwin R., Sparks Charles P., General Psychology, 4th ed. (New York: Barnes & Noble, 1954), p. 155.