Hans Eysenck was a psychologist who examined personality by using factor analysis, a statistical method. He concluded that people have three main personality factors that account for differences and variations in personality.
The first factor Eysenck referred to as Neuroticism (N) and it measured tendency to experience negative emotions. If a person scored high on the N scale, he/she may suffer from anxiety problems.
The second factor Eysenck referred to as Extroversion (E) and it measured tendency to experience positive social emotions. If a person scored low on the E scale, he/she may be socially reclusive.
The third factor Eysenck named Psychoticism (P) and it measured at one end aggressivenes/ divergent thinking and at the other end empathy/caution.
The acronym for his Three Factor Model is PEN.
P Psychoticism-Self Control
E Extroversion - Introversion
N Neuroticism - Emotional Stability
The major alternative to Eysenck's Three Factor Model of personality is the Big Five Model of personality.