For example, Jack talks to others because he is high on the trait of sociability, but we know that he is high on this trait because we observe that he spends a lot of time talking to others. The basis for Eysenck's emphasis on measurement and the development of a classification of traits is the statistical technique of factor analysis. Factor analysis is a technique in which one starts with a large number of test items that are administered to many people. Through a number of statistical procedures, clusters or factors are derived , the items within any single factor being highly related to one another and being only slightly related, or not at all related, to items in the other factors. According to trait theory, there are natural structures in personality, and factor analysis allows us to detect them.
Factor analysis assumes that behaviors that function with one another are related.
It is a statistical device for determining which behaviors are related to one another and thereby for determining the unities or natural elements in personality. The process described leads to factors, in this case called traits, that are named in terms of the characteristic that seems common to the items or behaviors found to be related to one another. Through some further statistical procedures Eysenck determines the basic dimensions that underlie the factors or traits that have been found. These basic dimensions are called types. Thus, for example, the traits of sociability, impulsiveness, activity, liveliness, and excitability can be grouped together under the type concept of extraversion.