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Moving towards meaningful measurement: Rasch analysis of the North Star Ambulatory Assessment in Duchenne muscular dystrophy

Lookup NU author(s): Dr Anna Mayhew, Dr Michelle Eagle, Emerita Professor Katherine Bushby, Professor Volker Straub, Dr Anna Sarkozy, Dr Eugen-Matthias Strehle

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Abstract

Aim 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. Method NSAA data from 191 ambulant boys (mean age at assessment 7y 8mo, SD 2y 4mo; range 3y 6mo-15y 5mo) 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. Interpretation 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.


Publication metadata

Author(s): Mayhew A, Cano S, Scott E, Eagle M, Bushby K, Muntoni F, Straub V (collaborator), Sarkozy A (collaborator), Strehle E (collaborator), Venkateswaran R (collaborator), North Star Clinical Network for Paediatric Neuromuscular Disease

Publication type: Article

Publication status: Published

Journal: Developmental Medicine and Child Neurology

Year: 2011

Volume: 53

Issue: 6

Pages: 535-542

Print publication date: 17/03/2011

ISSN (print): 0012-1622

ISSN (electronic): 1469-8749

Publisher: Mac Keith Press

URL: http://dx.doi.org/10.1111/j.1469-8749.2011.03939.x

DOI: 10.1111/j.1469-8749.2011.03939.x

Notes: A. Mayhew, S. Cano, E. Scott, M. Eagle, K. Bushby and F. Muntoni = main authors; V. Straub, A. Sarkozy, E. Strehle and R. Venkateswaran = collaborators in the North Star Clinical Network for Paediatric Neuromuscular Disease and members of staff of the Institute of Human Genetics, Newcastle University


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