title: Using Rasch analysis to compare the psychometric properties of the Short Form 36 physical function score and the Health Assessment Questionnaire disability index in patients with psoriatic arthritis and rheumatoid arthritis.
abstract: OBJECTIVE
  Item-response theory is increasingly used in the development of robust measurement tools. The extent to which the Health Assessment Questionnaire (HAQ) disability index (DI) and Short Form 36 (SF-36) physical functioning subscale (PF) fit a Rasch model in psoriatic arthritis (PsA) is uncertain. Our objective was to compare the psychometric properties of the HAQ DI and SF-36 PF in PsA and rheumatoid arthritis (RA) using Rasch analysis.
  
  
  METHODS
  Patients with RA (n = 142) and PsA (n = 134) were identified from a disease register based at a regional rheumatology service that serves a population of approximately 400,000 individuals. Responses to the HAQ DI and SF-36 PF were analyzed for item fit, differential item functioning (DIF), scale length (item separation), floor effects, and item difficulty by fitting the data to a Rasch model. The extent to which each instrument measured the same concept (disability) was also assessed in the PsA cohort using the Rasch model.
  
  
  RESULTS
  Item separation was much better for the SF-36 PF than the HAQ DI in PsA (9.12 logits versus 2.06 logits). There was evidence of marked DIF for the HAQ DI items activities, grip, and rising and relatively minor DIF for 4 items of the SF-36 PF. The distribution of SF-36 PF was better than HAQ DI in PsA, with floor effects of 3.1% versus 30.4%. Common person equating demonstrated that the 2 instruments measure the same construct in PsA.
  
  
  CONCLUSIONS
  The SF-36 PF has significant psychometric advantages over the HAQ DI in PsA.
authors.affiliation: Wellington School of Medicine and Health Sciences, University of Otago, Newtown, Wellington, New Zealand. will.taylor@otago.ac.nz
authors.forename: William J
authors.initials: WJ
authors.lastname: Taylor
authors.forename: Kathryn M
authors.initials: KM
authors.lastname: McPherson
mesh_terms: D000203:Activities of Daily Living; D015535:Arthritis, Psoriatic / Q000175:diagnosis / Q000503:physiopathology / Q000523:psychology*; D001172:Arthritis, Rheumatoid / Q000175:diagnosis / Q000503:physiopathology / Q000523:psychology*; D004185:Disability Evaluation; D005163:Factor Analysis, Statistical; D005260:Female; D006304:Health Status; D006801:Humans; D008297:Male; D008875:Middle Aged; D011594:Psychometrics / Q000706:statistics & numerical data; D015203:Reproducibility of Results; D012219:Rheumatology / Q000379:methods* / Q000706:statistics & numerical data; D012720:Severity of Illness Index; D011795:Surveys and Questionnaires