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For a number of years Thurstone has been attempting to analyze intellectual capacity into a number of elementary factors. His method has been to give a wide range of psychological tests to a large number of individuals and then to subject the correlations between the test scores to mathematical analysis.
Thurstone's mathematical methods and the assumptions upon which they are based were described in detail in The Vectors of Mind published in 1935. Primary Mental Abilities contains a considerably less technical resumé of these methods followed by a brief summary of some of the results of the author's research.
By purely mathematical methods it is possible to determine a minimum number of mathematically independent elementary factors necessary to account for the correlations and differences between the test results. The mathematical analysis indicates that an individual's rating in one of these elementary mental abilities will contribute in high proportion (say sixty percent, for example) to his score in a particular test, whereas another elementary mental ability will contribute in much lower proportion (say twenty percent) and still others will contribute not at all to this particular test. Up to this point the analysis has been based on a purely mathematical treatment of the test results with careful avoidance of all psychological preconceptions. In order to emphasize this avoidance of psychological preconceptions, Thurstone has chosen to designate his primary mental abilities by letters instead of names. The psychological analysis follows upon the completion of the mathematical matical analysis. All tests to which a given factor makes a particularly high contribution are then compared and the attempt is made to find psychological elements common to these tests.
The factors isolated by Thurstone are listed as follows with their probable psychological content so far as this can be deduced from comparison of the tests that have this particular factor in common:
S