title: Moving towards meaningful measurement: Rasch analysis of the North Star Ambulatory Assessment in Duchenne muscular dystrophy.
abstract: OBJECTIVE
  Reliable measurement of disease progression and the effect of therapeutic interventions in Duchenne muscular dystrophy (DMD) require clinically meaningful and scientifically sound rating scales. Therefore, we need robust evidence to support such tools. The North Star Ambulatory Assessment (NSAA) is a promising, clinician-rated scale with potential uses spanning clinical practice and clinical trials. In this study, we used Rasch analysis to test its suitability in these roles as a measurement instrument.
  
  
  METHODS
  NSAA data from 191 ambulant boys (mean age at assessment 7 y 8 mo, SD 2 y 4 mo; range 3 y 6 mo-15 y 5 mo) with a confirmed diagnosis of DMD were examined for psychometric properties including clinical meaning, targeting, response categories, model fit, reliability, dependency, stability, and raw to interval-level measurement. All analyses were performed using the Rasch Unidimensional Measurement Model.
  
  
  RESULTS
  Overall, Rasch analysis supported the NSAA as being a reliable (high Person Separation Index of 0.91) and valid (good targeting, little misfit, no reversed thresholds) measure of ambulatory function in DMD. One item displayed misfit (lifts head, fit residual 6.9) and there was evidence for some local dependency (stand on right/left leg, climb and descend box step right/left leg, and hop on right/left leg, residual correlations >0.40), which we provide potential solutions for in future use of the NSAA. Importantly, our findings supported good clinical validity in that the hierarchy of items within the scale produced by the analyses was supported by clinical opinion, thus increasing the clinical interpretability of scale scores.
  
  
  CONCLUSIONS
  In general, Rasch analysis supported the NSAA as a psychometrically robust scale for use in DMD clinical research and trials. This study also demonstrates how Rasch analysis is a useful instrument to detect and understand the key measurement issues of rating scales.
authors.affiliation: Institute of Human Genetics, International Centre for Life, Newcastle University, Newcastle upon Tyne, UK. anna.mayhew@ncl.ac.uk
authors.forename: Anna
authors.initials: A
authors.lastname: Mayhew
authors.forename: Stefan
authors.initials: S
authors.lastname: Cano
authors.forename: Elaine
authors.initials: E
authors.lastname: Scott
authors.forename: Michelle
authors.initials: M
authors.lastname: Eagle
authors.forename: Kate
authors.initials: K
authors.lastname: Bushby
authors.forename: Francesco
authors.initials: F
authors.lastname: Muntoni
mesh_terms: D000293:Adolescent; D016009:Chi-Square Distribution; D002648:Child; D002675:Child, Preschool; D004185:Disability Evaluation; D005080:Exercise Test / Q000379:methods; D005260:Female; D006801:Humans; D008297:Male; D020388:Muscular Dystrophy, Duchenne / Q000175:diagnosis* / Q000503:physiopathology* / Q000534:rehabilitation; D011336:Probability; D011446:Prospective Studies; D011594:Psychometrics / Q000379:methods*; D016138:Walking