1. Bocher, Maxime, Introduction to Higher Algebra, New York: Macmillan, 1907, 1921. pp. xi + 321. Chapter VI.
2. Hotelling, Harold, Analysis of a Complex of Statistical Variables into Principal Components.J. Educ. Psychol., 1933, 24, 417–441, 498–520.
3. Hotelling, Harold, The Most Predictable Criterion,J. Educ. Psychol., 1935, 26, 139–142.
4. Kelley, Truman L., Crossroads in the Mind of Man. Stanford University: Stanford University Press, 1928. pp. 238. p. 100.
5. Kelley, Truman L., Essential Traits of Mental Life. Cambridge: Harvard University Press, 1935. pp. 145.