title: Evaluation of the Multiple Sclerosis Walking Scale-12 (MSWS-12) in a Dutch sample: Application of item response theory.
abstract: The Multiple Sclerosis Walking Scale-12 (MSWS-12) measures walking ability from the patients' perspective. We examined the quality of the MSWS-12 using an item response theory model, the graded response model (GRM).
  
  
  
  A total of 625 unique Dutch multiple sclerosis (MS) patients were included. After testing for unidimensionality, monotonicity, and absence of local dependence, a GRM was fit and item characteristics were assessed. Differential item functioning (DIF) for the variables gender, age, duration of MS, type of MS and severity of MS, reliability, total test information, and standard error of the trait level (θ) were investigated.
  
  
  
  Confirmatory factor analysis showed a unidimensional structure of the 12 items of the scale, explaining 88% of the variance. Item 2 did not fit into the GRM model. Reliability was 0.93. Items 8 and 9 (of the 11 and 12 item version respectively) showed DIF on the variable severity, based on the Expanded Disability Status Scale (EDSS). However, the EDSS is strongly related to the content of both items.
  
  
  
  Our results confirm the good quality of the MSWS-12. The trait level (θ) scores and item parameters of both the 12- and 11-item versions were highly comparable, although we do not suggest to change the content of the MSWS-12.
authors.affiliation: Department of Epidemiology and Biostatistics, VU University Medical Center, Amsterdam, The Netherlands w.mokkink@vumc.nl.
authors.forename: Lidwine Brigitta
authors.initials: LB
authors.lastname: Mokkink
authors.affiliation: Department of Epidemiology and Biostatistics, VU University Medical Center, Amsterdam, The Netherlands.
authors.forename: Francisca
authors.initials: F
authors.lastname: Galindo-Garre
authors.affiliation: Department of Neurology, VU University Medical Center, Amsterdam, The Netherlands/VUmc MS Center Amsterdam, The Netherlands.
authors.forename: Bernard Mj
authors.initials: BM
authors.lastname: Uitdehaag
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009103:Multiple Sclerosis / Q000175:diagnosis*; D009426:Netherlands; D011594:Psychometrics / Q000295:instrumentation*; D015203:Reproducibility of Results; D057566:Self Report / Q000592:standards*; D012720:Severity of Illness Index; D016138:Walking; D055815:Young Adult
keywords: Multiple Sclerosis Walking Scale-12 (MSWS-12); differential item functioning; graded response model; item fit; item response theory (IRT); multiple sclerosis; psychometrics; walking ability