1. Grant, E. L. and Leavenworth, R. S. (1980). Statistical Quality Control. McGraw-Hill Book Company, New York, NY, p. 91.
2. The Shewhart Control Chart—Tests for Special Causes
3. Ott, E. R. (1975). Process Quality Control. McGraw-Hill Book Company, New York, NY, pp. 35–36.
4. Shewhart, W. A. (1931). Economic Control of Quality of Manufactured Product. Reprinted in 1980 by the American Society for Quality Control, pp. 304–318.