Norwegian reference data on the Fatigue Questionnaire and the Patient Health Questionnaire-9 and their interrelationship.

This study showed considerable interrelationship between the FQ and the PHQ-9 constructs. The reference data show that scores on the FQ have only improved significantly in persons aged 60 or more years between 1996 and 2015. Our prevalence findings of MDE based on the PHQ-9 are in accordance with the findings from other countries. The FQ and the PHQ-9 should be used together in epidemiological and clinical studies.