title: How useful are the SF-36 sub-scales in older people? Mokken scaling of data from the HALCyon programme.
abstract: OBJECTIVE
  To evaluate two psychometric properties of SF-36, namely unidimensionality and reliability.
  
  
  METHODS
  The data are from three cohorts in the HALCyon collaborative research programme into healthy ageing: Aberdeen Birth Cohort 1936 (n = 428), Hertfordshire Ageing Study (n = 358) and Hertfordshire Cohort Study (n = 3,216). The Mokken scaling model was applied to each sub-scale of SF-36 to evaluate unidimensionality as indicated by scalability. The lower bound for internal consistency reliability was determined by Cronbach's alpha.
  
  
  RESULTS
  All six sub-scales of SF-36, with the exception of general health (GH) and mental health (MH), demonstrated strong scalability (0.5 ≤ H < 1). The results were consistent across all 3 cohorts. Both GH and MH showed medium scalability (0.4 ≤ H <0.55), although individual items 'sick easier..', 'as healthy as..' and 'expect to get worse' of the GH sub-scale and 'nervous', 'happy' in the MH sub-scale had low scalability (H < 0.4) in the oldest cohort (aged 73-83). Cronbach's alphas for all sub-scales were between 0.70 and 0.92.
  
  
  CONCLUSIONS
  The unidimensionality and reliability of the sub-scales of SF-36 are sufficient to make this a useful measure of health-related quality of life in older people. Caution is needed when interpreting the results for GH and MH in the oldest cohort due to the poor unidimensionality.
authors.affiliation: MRC Unit for Lifelong Health and Ageing, University College London, 33, Bedford Place, London, WC1B 5JU, UK. g.mishra@nshd.mrc.ac.uk
authors.forename: Gita D
authors.initials: GD
authors.lastname: Mishra
authors.forename: Catharine R
authors.initials: CR
authors.lastname: Gale
authors.forename: Avan Aihie
authors.initials: AA
authors.lastname: Sayer
authors.forename: Cyrus
authors.initials: C
authors.lastname: Cooper
authors.forename: Elaine M
authors.initials: EM
authors.lastname: Dennison
authors.forename: Lawrence J
authors.initials: LJ
authors.lastname: Whalley
authors.forename: Leone
authors.initials: L
authors.lastname: Craig
authors.forename: Diana
authors.initials: D
authors.lastname: Kuh
authors.forename: Ian J
authors.initials: IJ
authors.lastname: Deary
mesh_terms: D000368:Aged; D015331:Cohort Studies; D005260:Female; D006304:Health Status; D006306:Health Surveys / Q000295:instrumentation*; D006801:Humans; D008297:Male; D008875:Middle Aged; D011594:Psychometrics / Q000295:instrumentation*; D011788:Quality of Life; D015203:Reproducibility of Results; D012606:Scotland; D013223:Statistics as Topic / Q000379:methods; D011795:Surveys and Questionnaires / Q000592:standards*