L. L. Thurstone's study presents the first major application of factor analysis to psychological testing, revealing primary mental abilities. The research involved 240 volunteers and 56 psychological tests, aiming to identify fundamental cognitive and conative traits. The study found that primary factors, such as induction and deduction, differ from initial tentative categories. The analysis showed that tests often collapsed into a single visual space factor, and that reasoning abilities transcend the immediate content of the tests. The study highlights the effectiveness of factorial methods in testing psychological hypotheses and emphasizes the need for more refined tests in future research. The results suggest that primary mental abilities can be assessed through simple perceptual tasks, potentially replacing traditional paper-pencil tests. The study also notes that the general factor of Spearman was not found, but further research may confirm its existence. The research underscores the importance of factorial analysis in isolating primary mental abilities and provides a foundation for future studies in experimental psychology. The study's findings contribute to the understanding of cognitive and conative traits, and the results indicate that individual testing with specially designed materials may be more effective than group testing. The research also emphasizes the importance of continued experimental studies to refine and validate the identification of primary mental abilities. The study concludes that factorial methods are essential for understanding the structure of mental abilities and that future research will build on these findings to develop more accurate and effective psychological tests.L. L. Thurstone's study presents the first major application of factor analysis to psychological testing, revealing primary mental abilities. The research involved 240 volunteers and 56 psychological tests, aiming to identify fundamental cognitive and conative traits. The study found that primary factors, such as induction and deduction, differ from initial tentative categories. The analysis showed that tests often collapsed into a single visual space factor, and that reasoning abilities transcend the immediate content of the tests. The study highlights the effectiveness of factorial methods in testing psychological hypotheses and emphasizes the need for more refined tests in future research. The results suggest that primary mental abilities can be assessed through simple perceptual tasks, potentially replacing traditional paper-pencil tests. The study also notes that the general factor of Spearman was not found, but further research may confirm its existence. The research underscores the importance of factorial analysis in isolating primary mental abilities and provides a foundation for future studies in experimental psychology. The study's findings contribute to the understanding of cognitive and conative traits, and the results indicate that individual testing with specially designed materials may be more effective than group testing. The research also emphasizes the importance of continued experimental studies to refine and validate the identification of primary mental abilities. The study concludes that factorial methods are essential for understanding the structure of mental abilities and that future research will build on these findings to develop more accurate and effective psychological tests.