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Improving measurement properties of the PHQ-9 with rating scale analysis.


Williams, R. , Heinemann, A. , Bode, R., Wilson, C. , Fann, J., & Tate, D



Publication Info:

Rehabilitation Psychology, 54(2):198-203


OBJECTIVE: The goal of this study was to explore the psychometric properties of the Patient Health Questionnaire-9 (PHQ-9; R. L. Spitzer, K. Kroenke, & J. B. W. Williams, 1999).
METHOD: Factor analysis and Rasch rating scale analysis were used to examine the psychometric properties of the PHQ-9. The sample consisted of 202 adults with spinal cord injury (SCI).
RESULTS: The PHQ-9 items appear to form a usefully unidimensional scale. One "double-barreled" item, "Moving or speaking so slowly that other people could have noticed or being so fidgety or restless that you've been moving around a lot more than usual," misfit the Rasch model. Category probability curves indicate respondent difficulty in distinguishing between the 2 intermediate rating scale categories: several days and more than half the days. Combining these categories eliminated this problem and resulted in all items fitting the measurement model.
CONCLUSIONS: The measurement properties of the PHQ-9 can be improved by collapsing rating scale categories and by restructuring several double- and triple-barreled items. Adopting these changes may improve sensitivity in measuring depression after SCI.

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