As I proposed in my volume Human Cognitive Abilities (Carroll 1993), cognitive abilities may be assumed to exist at three principal levels or strata: a first, lower-order stratum comprising some 50 to 60 or more narrow abilities that are linearly independent of each other (that is, possibly inter-correlated but with clearly separated vectors in the factorial space); a second stratum comprising approximately 8 to 10 or more broad abilities, also linearly independent of each other; and a third still higher stratum containing only a single, general intellectual ability commonly termed g.