Editing Principal component analysis (section)

=== Intelligence ===
The earliest application of factor analysis was in locating and measuring components of human intelligence. It was believed that intelligence had various uncorrelated components such as spatial intelligence, verbal intelligence, induction, deduction etc and that scores on these could be adduced by factor analysis from results on various tests, to give a single index known as the [[Intelligence quotient|Intelligence Quotient]] (IQ). The pioneering statistical psychologist [[Charles Spearman|Spearman]] actually developed factor analysis in 1904 for his [[Two-factor theory of intelligence|two-factor theory]] of intelligence, adding a formal technique to the science of [[psychometrics]]. In 1924 [[Louis Leon Thurstone|Thurstone]] looked for 56 factors of intelligence, developing the notion of Mental Age. Standard IQ tests today are based on this early work.<ref name="Kaplan, R.M. 2010">Kaplan, R.M., & Saccuzzo, D.P. (2010). ''Psychological Testing: Principles, Applications, and Issues.'' (8th ed.). Belmont, CA: Wadsworth, Cengage Learning.</ref>