1. Anastasi A., Psychological Testing, p. 104, MacMillan, New York, 1968.
2. Harman H. H., Modern Factor Analysis, pp. 6–7, University of Chicago Press, Chicago, 1967.
3. Brown F. G., Principles of Educational and Psychological Testing, pp. 220–221, Dryden Press, Hinsdale, Illinois, 1970.
4. The Application of Electronic Computers to Factor Analysis