theories of intelligence based on or tested by scores on conventional tests of intelligence, such as number-series completions and verbal analogies. These theories are often, but not always, based on factor analysis; that is, they specify a set of factors alleged to underlie human intelligence. Among the most famous of such theories are Charles Spearman’s two-factor theory and Louis L. Thurstone’s theory of primary abilities. See also radex theory of intelligence; three-stratum model of intelligence.