Thurstone (1938) suggested that intelligence is a composite of seven distinct primary mental abilities (PMA). He analyzed inter correlations of a set of 56 mental tests, and identified seven factors, each of which he called a primary mental ability.
Thurstone considered each of the mental abilities to be independent of other, and that each can be assessed separately. He believed that there is single score of an aggregate intelligence. His Primary Mental Abilities 1 (PMA) is widely used.