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Differential item functioning in the Observer Scale of the POSAS for different scar types

Journal article
Authors M. B. A. van der Wal
W. E. Tuinebreijer
Åsa Lundgren Nilsson
E. Middelkoop
P. P. M. van Zuijlen
Published in Quality of Life Research
Volume 23
Issue 7
Pages 2037-2045
ISSN 0962-9343
Publication year 2014
Published at Institute of Neuroscience and Physiology, Department of Clinical Neuroscience and Rehabilitation
Pages 2037-2045
Language en
Keywords Differential item functioning, POSAS, Rasch, Scar quality, Scar, assessment, RANDOMIZED CONTROLLED-TRIAL, RASCH ANALYSIS, HYPERTROPHIC SCARS, BURN, SCARS, ASSESSMENT TOOLS, SILICONE GEL, PATIENT, INDEX, MATURATION, DISEASES, Health Care Sciences & Services, Health Policy & Services, Public,, Environmental & Occupational Health
Subject categories Public Health, Global Health, Social Medicine and Epidemiology


To investigate whether the Observer Scale of the Patient and Observer Scar Assessment Scale (POSAS) can serve as a generic measure for scar quality across different scar types. A collection of POSAS scores derived from several clinical trials on burn (n = 404), linear (n = 384), and keloidal scars (n = 282) was analyzed using the partial credit model of the Rasch analysis package RUMM2030. Differential item functioning (DIF) was observed for the Observer Scale of the POSAS between the three scar types for the items pliability, thickness, and surface area, which could be solved by item splitting. The items pigmentation and thickness showed disordered thresholds, considerable misfit, and unpredictability. Users of the Observer Scale of the POSAS must be aware that the raw scores obtained from burn, linear, and keloidal scars cannot be compared without the scar-specific DIF adjustment of the items pliability, thickness, and surface area.

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