Evaluating the Psychometric Properties of the German Self and Interpersonal Functioning Scale (SIFS)

The Self and Interpersonal Functioning Scale (SIFS) is a 24-item self-report questionnaire assessing personality functioning according to the alternative DSM-5 model for personality disorders. We evaluated the German SIFS version in a total sample of 886 participants from Germany and Switzerland. Its factor structure was investigated with confirmatory factor analysis comparing bifactor models with two specific factors (self- and interpersonal functioning) and four specific factors (identity, self-direction, empathy, and intimacy). The SIFS sum and domain scores were tested for reliability and convergent validity with self-report questionnaires and interviews for personality functioning, -organization, -traits, -disorder categories, and well-being. None of the bifactor models yielded good model fit, even after excluding two items with low factor loadings and including a method factor for reverse-keyed items. Based on a shortened 22-item SIFS version, models suggested that the g-factor explained 52.9-59.6% of the common variance and that the SIFS sum score measured the g-factor with a reliability of .68-.81. Even though the SIFS sum score showed large test-retest reliability and correlated strongly with well-established self-report questionnaires and interviews, the lack of structural validity appears to be a serious disadvantage of the SIFS compared to existing self-reports questionnaires of personality functioning.