Item format and the psychometric properties of the Eysenck Personality Questionnaire

Most psychometric questionnaires used for evaluating personality traits are developed utilizing dichotomous item formats. From a psychometric viewpoint the Likert-type item format has more advantages than the dichotomous format. If so, why is the dichotomous format so widely used in the framework of personality tests? Does empirical evidence support this extensive use? The main goal of this research is to study systematically the way in which the number of categories of the Likert-type item format used (from two to nine categories) affects the main psychometric properties of the scale: reliability and validity. The Eysenck Personality Questionnaire was used, changing its original yes/no format into a Likert-type scale, from dichotomous to nine categories. A sample of 1149 participants (578 men and 571 women) was used. The results show that the psychometric properties of the test (reliability and validity) improve as the number of item categories is raised. Seven categories maximize the psychometric properties of the test. The implications of these findings for Eysenck's personality model are analyzed, and some considerations for practitioners and professionals discussed.