title: Assessing items on the SF-8 Japanese version for health-related quality of life: a psychometric analysis based on the nominal categories model of item response theory.
abstract: OBJECTIVE
  The Short Form-8 (SF-8) questionnaire is a commonly used 8-item instrument of health-related quality of life (QOL) and provides a health profile of eight subdimensions. Our aim was to examine the psychometric properties of the Japanese version of the SF-8 instrument using methodology based on nominal categories model.
  
  
  METHODS
  Using data from an adjusted random sample from a nationally representative panel, the nominal categories modeling was applied to SF-8 items to characterize coverage of the latent trait (theta). Probabilities for response choices were described as functions on the latent trait. Information functions were generated based on the estimated item parameters.
  
  
  RESULTS
  A total of 3344 participants (53%, women; median age, 35 years) provided responses. One factor was retained (eigenvalue, 4.65; variance proportion of 0.58) and used as theta. All item response category characteristic curves satisfied the monotonicity assumption in accurate order with corresponding ordinal responses. Four items (general health, bodily pain, vitality, and mental health) cover most of the spectrum of theta, while the other four items (physical function, role physical [role limitations because of physical health], social functioning, and role emotional [role limitations because of emotional problems] ) cover most of the negative range of theta. Information function for all items combined peaked at -0.7 of theta (information = 18.5) and decreased with increasing theta.
  
  
  CONCLUSIONS
  The SF-8 instrument performs well among those with poor QOL across the continuum of the latent trait and thus can recognize more effectively persons with relatively poorer QOL than those with relatively better QOL.
authors.affiliation: Center for Clinical Epidemiology, St. Luke's International Hospital, Tokyo, Japan. tokuyasu@orange.ocn.ne.jp
authors.forename: Yasuharu
authors.initials: Y
authors.lastname: Tokuda
authors.forename: Tomoya
authors.initials: T
authors.lastname: Okubo
authors.forename: Sachiko
authors.initials: S
authors.lastname: Ohde
authors.forename: Joshua
authors.initials: J
authors.lastname: Jacobs
authors.forename: Osamu
authors.initials: O
authors.lastname: Takahashi
authors.forename: Fumio
authors.initials: F
authors.lastname: Omata
authors.forename: Haruo
authors.initials: H
authors.lastname: Yanai
authors.forename: Shigeaki
authors.initials: S
authors.lastname: Hinohara
authors.forename: Tsuguya
authors.initials: T
authors.lastname: Fukui
mesh_terms: D000328:Adult; D003142:Communication; D003469:Culture; D005260:Female; D006801:Humans; D007564:Japan; D007802:Language; D008297:Male; D008960:Models, Psychological; D011594:Psychometrics; D011788:Quality of Life; D015203:Reproducibility of Results; D011795:Surveys and Questionnaires