Modelling non-normal data: The relationship between the skew-normal factor model and the quadratic factor model.

Maximum likelihood estimation of the linear factor model for continuous items assumes normally distributed item scores. We consider deviations from normality by means of a skew-normally distributed factor model or a quadratic factor model. We show that the item distributions under a skew-normal factor are equivalent to those under a quadratic model up to third-order moments.

Cohort differences in Big Five personality factors over a period of 25 years.

The notion of personality traits implies a certain degree of stability in the life span of an individual. But what about generational effects? Are there generational changes in the distribution or structure of personality traits? This article examines cohort changes on the Big Five personality factors Extraversion, Agreeableness, Conscientiousness, Neuroticism, and Openness to Experience, among first-year psychology students in The Netherlands, ages 18 to 25 years, between 1982 and 2007. Because measurement invariance of a personality test is essential for a sound interpretation of cohort differences in personality, we first assessed measurement invariance with respect to cohort for males and females separately on the Big Five personality factors, as measured by the Dutch instrument Five Personality Factors Test.