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The three-stratum theory is a theory of cognitive ability proposed by the American psychologist John Carroll in 1993. It is based on a factor-analytic study of the correlation of individual-difference variables from data such as psychological tests, school marks and competence ratings. These analyses suggested a three-layered model where each layer accounts for the variations in the correlations within the previous layer.
The three layers (strata) are defined as representing narrow, broad, and general cognitive ability. The factors describe stable and observable differences among individuals in the performance of tasks. Carroll argues further that they are not mere artifacts of a mathematical process, but likely reflect physiological factors explaining differences in ability (e.g., nerve firing rates). This of course does not alter the effectiveness of factor scores in accounting for behavioral differences.
Carroll proposes a taxonomic dimension in the distinction between level factors and speed factors. The tasks that contribute to the identification of level factors can be sorted by difficulty and individuals differentiated by whether they have acquired the skill to perform the tasks. Tasks that contribute to speed factors are distinguished by the relative speed with which individuals can complete them. Carroll suggests that the distinction between level and speed factors may be the broadest taxonomy of cognitive tasks that can be offered. Carroll distinguishes his hierarchical approach from taxonomic approaches such as Guilford’s Structure of Intellect model (three-dimensional model with contents, operations, and products).