Improving measurement properties of the Patient Health Questionnaire-9 with rating scale analysis.
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).


METHODS
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.
Williams
Ryan T
RT
Center for Rehabilitation Outcomes Research, Rehabilitation Institute of Chicago. rwilliams@ric.org
Heinemann
Allen W
AW
Bode
Rita K
RK
Wilson
Catherine S
CS
Fann
Jesse R
JR
Tate
Denise G
DG
D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D003866:Depressive Disorder / Q000175:diagnosis* / Q000523:psychology; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D011594:Psychometrics / Q000706:statistics & numerical data; D015203:Reproducibility of Results; D013119:Spinal Cord Injuries / Q000523:psychology* / Q000534:rehabilitation*; D011795:Surveys and Questionnaires; D055815:Young Adult