1. Walker, Helen M., Elementary Statistical Methods (New York: Henry Holt, 1943), pp. 201–21.
2. Rider, Paul R. An Introduction to Modern Statistical Methods (New York: John Wiley & Sons, Inc., 1939), pp. 27, 47–50.
3. Blair, Morris Myers. Elementary Statistics with General Applications (New York: Henry Holt, 1944), pp. 229–60, 264–84.