title: Dimensional assessment of depressive severity in the elderly general population: psychometric evaluation of the PHQ-9 using Rasch Analysis.
abstract: BACKGROUND
  The depression module of the Patient Health questionnaire (PHQ-9) is a wide-spread self-report instrument for the assessment of depression with compelling psychometric characteristics when relying on classical test theory assumptions. This study aimed at evaluating whether the PHQ-9 may be interpreted as a dimensional scale measuring depression severity in the elderly general population using Rasch analysis with special emphasis on its unidimensional structure and differential item functioning (DIF) due to gender, age, and the presence of somatic multimorbidity.
  
  
  METHODS
  A representative sample of the elderly German general population (N=1631; age 60-85 years, 53.5% female) filled in the PHQ-9, a questionnaire about chronic medical conditions and a demographic data sheet. Unidimensionality and psychometric properties of the PHQ-9 were ascertained applying confirmatory factor analysis (CFA) and Rasch analysis.
  
  
  RESULTS
  Results revealed substantial violations of the unidimensionality of the scale: item 8 (retardation or agitation) had to be eliminated and multiple residual correlations were added. Gender-related DIF emerged for two items, and three items showed insufficient Rasch model fit.
  
  
  CONCLUSIONS
  The large sample leads to high statistical power that might technically increase the probability of detecting model misfit or DIF. The sampling procedure leads to a possible underestimation of morbidity due to the exclusion of those elderly patients living in nursing homes.
  
  
  CONCLUSIONS
  Results suggest that - when applied in the elderly general population - the PHQ-9 should be interpreted in terms of a diagnostic algorithm for classificatory decisions about a DSM-IV based probable diagnosis of depression rather than as a dimensional scale.
authors: Institute of Medical Psychology and Medical Sociology, University Hospital of RWTH Aachen, Pauwelsstraße 19, 52074, Aachen, Germany.
authors: Thomas
authors: T
authors: Forkmann
authors: Siegfried
authors: S
authors: Gauggel
authors: Lena
authors: L
authors: Spangenberg
authors: Elmar
authors: E
authors: Brähler
authors: Heide
authors: H
authors: Glaesmer
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D003863:Depression / Q000175:diagnosis* / Q000523:psychology; D039721:Diagnostic and Statistical Manual of Mental Disorders; D005163:Factor Analysis, Statistical; D005260:Female; D005858:Germany; D006801:Humans; D008297:Male; D008875:Middle Aged; D011569:Psychiatric Status Rating Scales; D011594:Psychometrics; D015203:Reproducibility of Results; D012720:Severity of Illness Index; D011795:Surveys and Questionnaires