The Patient Health Questionnaire-9 Item (PHQ-9) and Beck Depression Inventory-II (BDI-II) are frequently used measures of depression severity, but little is known about their relative psychometric properties. The authors assessed psychometric properties of both measures during treatment for depression. The PHQ-9 and BDI-II scores from 172 depressed participants in two randomized controlled trials of treatment for depression were assessed and combined. Tests of internal consistency (Cronbach's α), factor analyses, correlational analyses, estimates of clinically significant change, and effect sizes (Cohen's d) were calculated after treatment and follow-up. Both scales demonstrated adequate internal consistency at pre- and posttreatment (PHQ-9 α = .74 and .81; BDI-II α = .87 and .90, respectively). Factor analysis failed to confirm the one-factor model previously reported for the PHQ-9, but two factors evidenced good fit for the BDI-II. Both scales converged more with each other than with the Sheehan Disability Scale at pre- but not at posttreatment. Responsiveness to change of PHQ-9 and BDI-II was similar at both posttreatment and follow-up. The consistency of agreement on indices of clinical significance was fair to moderate, but the BDI-II categorised a greater proportion of participants with severe depression than the PHQ-9. The BDI-II and PHQ-9 demonstrated adequate reliability, convergent/discriminant validity, and similar responsiveness to change. Differences were found in how they categorised severity. Pending the results of further studies, the attributes of the PHQ-9, of being shorter and based on the diagnostic criteria for depression, may indicate an advantage over the BDI-II.
Keywords: BDI-II; PHQ-9; depression; psychometric properties; treatment.