title: Standardization of depression measurement: a common metric was developed for 11 self-report depression measures.
abstract: OBJECTIVE
  To provide a standardized metric for the assessment of depression severity to enable comparability among results of established depression measures.
  
  
  METHODS
  A common metric for 11 depression questionnaires was developed applying item response theory (IRT) methods. Data of 33,844 adults were used for secondary analysis including routine assessments of 23,817 in- and outpatients with mental and/or medical conditions (46% with depressive disorders) and a general population sample of 10,027 randomly selected participants from three representative German household surveys.
  
  
  RESULTS
  A standardized metric for depression severity was defined by 143 items, and scores were normed to a general population mean of 50 (standard deviation = 10) for easy interpretability. It covers the entire range of depression severity assessed by established instruments. The metric allows comparisons among included measures. Large differences were found in their measurement precision and range, providing a rationale for instrument selection. Published scale-specific threshold scores of depression severity showed remarkable consistencies across different questionnaires.
  
  
  CONCLUSIONS
  An IRT-based instrument-independent metric for depression severity enables direct comparisons among established measures. The "common ruler" simplifies the interpretation of depression assessment by identifying key thresholds for clinical and epidemiologic decision making and facilitates integrative psychometric research across studies, including meta-analysis.
authors.affiliation: Department of Psychosomatic Medicine and Psychotherapy, University Medical Center Hamburg-Eppendorf & Schön Klinik Hamburg Eilbek, Martinistr. 52, 20246 Hamburg, Germany & Dehnhaide 120, 22081 Hamburg, Germany.
authors.forename: Inka
authors.initials: I
authors.lastname: Wahl
authors.forename: Bernd
authors.initials: B
authors.lastname: Löwe
authors.forename: Jakob Bue
authors.initials: JB
authors.lastname: Bjorner
authors.forename: Felix
authors.initials: F
authors.lastname: Fischer
authors.forename: Gernot
authors.initials: G
authors.lastname: Langs
authors.forename: Ulrich
authors.initials: U
authors.lastname: Voderholzer
authors.forename: Stephen A
authors.initials: SA
authors.lastname: Aita
authors.forename: Niels
authors.initials: N
authors.lastname: Bergemann
authors.forename: Elmar
authors.initials: E
authors.lastname: Brähler
authors.forename: Matthias
authors.initials: M
authors.lastname: Rose
mesh_terms: D000328:Adult; D003863:Depression / Q000175:diagnosis*; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D011569:Psychiatric Status Rating Scales / Q000592:standards*; D011594:Psychometrics; D015203:Reproducibility of Results; D012720:Severity of Illness Index; D011795:Surveys and Questionnaires
keywords: Depression; Health outcome assessment; Item bank; Item response theory; Linking; Patient-reported outcomes