Three-stratum theory

[1][2] 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 from more than 460 datasets.

The three layers (strata) are defined as representing narrow, broad, and general cognitive ability.

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).

Carroll suggests that the distinction between level and speed factors may be the broadest taxonomy of cognitive tasks that can be offered.

[5] His factor analyses were largely consistent with the Horn-Cattell model except that Carroll believed that general intelligence was a meaningful construct.