As the search for distinct intellectual factors progressed, their number multiplied, and so did the number of models devised to organize them. One type of scheme, used by Cyril Burt, Philip Vernon, and others, is a hierarchical arrangement of factors. In these models, Spearman's g factor is placed at the top of a pyramid, and the specific factors are placed at the bottom. In between, there are one or more levels of group factors selected in terms of their breadth and arranged according to their interrelationships with the more general factors above them and the more specific factors below them.
In Vernon's scheme, for example, the ability to change a tire might be classified as a specific factor at the base of the pyramid, located underneath an intermediate group factor labeled mechanical information, which in turn would be under one of the two major group factors identified by Vernon as the main subdivisions under g—namely, the practical-mechanical factor. The hierarchical scheme for organizing mental abilities is a useful device that is endorsed by many psychologists on both sides of the Atlantic Ocean. It recognizes that very few tasks are so simple as to require a single skill for successful performance, that many intellectual functions have some common elements, and that some abilities play a more pivotal role than others in the performance of culturally valued activities.
Another well-known scheme for organizing intellectual traits is the structure-of-intellect (SOI) model developed byJ. P. Guilford. Although the SOI is grounded in extensive factor-analytic research conducted by Guilford throughout the 1940's and 1950's, the model goes beyond factor analysis and is perhaps the most ambitious attempt to classify systematically all the possible functions of the human intellect. The SOI classifies intellectual traits along three dimensions—namely, five types of operations, four types of contents, and six types of productions, for a total of 120 categories (5x4x6). Intellectual operations consist of what a person actually does (for example, evaluating or remembering something), the contents are the types of materials or information on which the operations are performed (for example, symbols, such as letters or numbers), and the products are the form in which the contents are processed (for example, units or relations).
Not all the 120 categories in Guilford's complex model have been used, but enough factors have been identified to account for about 100 of them, and some have proved very useful in labeling and understanding the skills that tests measure. Furthermore, Guilford's model has served to call attention to some dimensions of intellectual activity, such as creativity and interpersonal skills, that had been neglected previously.
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