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Val Tadic, Andrew Cooper, Gillian Lewando Hundt, Jugnoo Rahi, ; Development and validation of a novel functional vision instrument for children and young people with visual impairment. Invest. Ophthalmol. Vis. Sci. 2013;54(15):5686.
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Robust patient-reported outcome measures in paediatric ophthalmology are lacking. To complement our vision-related quality of life (VQoL) instrument for visually impaired (VI) children to self-report the impact of living with impaired vision, we have now developed a self-report measure of Functional Vision (FV). Here, we report development and validation of a novel FV instrument for VI (LogMAR worse than 0.48) children aged 10-15 years.
Qualitative interviews (n=32 VI children) supplemented by narrative feedback (n=15) were used to generate a draft FV instrument, with further review and reduction through individual consultations (n=17) about item relevance and comprehensibility, and instrument instructions, format and administration methods. The instrument was piloted with 94 VI children by postal survey to 21 NHS Trusts. Initial item reduction was guided by missing data and item distribution. Rasch Rating Scale Model, supplemented by Factor Analysis and internal reliability statistics, was applied to assess unidimensionality, precision, targeting and response category ordering.
712 qualitative statements were reduced to 56 items capturing difficulty in performing vision-dependent activities. Following the pilot, 8 items were removed based on high percentage of missing data, and a further 3 on the basis of skewness. 12 items were removed based on inadequate item infit and outfit values in Rasch. 11 items were removed, exhibiting differential item functioning across age and gender. The remaining 22-item scale showed infit and outfit values in Rasch within acceptable limits and the person-item map indicated good targeting of items to respondents. Visual inspection of the ICC plots for each item revealed ordered response categories. The reduced scale has high internal consistency (Chronbach Alpha=0.95) and a clear unidimensional structure.
Our novel 22-item FV instrument is a psychometrically robust measure for capturing the functional impact of visual disability from the child’s own perspective, complementing assessment of VQoL and offering an adjunct to clinical assessments in routine paediatric ophthalmology practice.
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