Trait theory is based on several assumptions. The first of many trait theory assumptions are that any difference between people that is seen as significant will have a name. These names, known as traits, are conceived of as continuous dimensions. Trait theories assume that people vary concurrently on a number of personality factors.
These traits are of both the conjunctive and disjunctive form. To understand a trait, it is necessary to understand what a particular trait is and what type of behavior is proof of that trait. As it became evident to many psychologists that, mathematically, combinations of five factors were useful in describing personality, there was a need to clearly define what these factors were. Indeed, this process led to some dissent in the ranks. One dissenter from the five-factor theorists was renowned psychologist H. J. Eysenck. Eysenck felt that, due to overlaps in the five factors and their correlates, in fact a three-factor model was more appropriate and accurate. His trait theory is called the PEN model (which stand for psychoticism, extroversion, neuroticism), or sometimes this trait is shortened to the two factor E-IN model (extroversion-introversion, neuroticism).
According to Eysenck, "Factor analysis has