Investigating the PHQ-9 With Mokken Scale Analysis and Cognitive Interviews.

Scores on the Patient Health Questionnaire-9 (PHQ-9) are frequently used to assess depression both in research and in clinical practice. The aim was to examine the validity of the PHQ-9 sum score by using Mokken scale analysis (Study I) and cognitive interviews (Study II) on the Icelandic version of PHQ-9. A primary care sample of 618 individuals was used in Study I.